Assorted WaveRenderer UI improvements

[fdxmw]

See the updated screenshots and tests for a preview:

• Add minor tick marks to rulers. This makes it easier to identify cycle boundaries
• Use different Unicode and ASCII characters for clearer display
  • Use different line drawing characters for more continuous lines
  • 'x' is a confusing separator in ASCII output as hex values also contain x's, use '|' instead
• When extra_line is enabled, draw additional major tick marks between signals
• Disable segment_delim by default as the additional major ticks show roughly the same thing in less horizontal space
• Make all segments the same length. Previously the first segment was one character longer

Also:

• Upgrade tox to use python3.8 and disable python2.7. Python 2.7 and 3.7 are currently not easy to install (at least on my macos machine)

[mdko]

I think all of this looks great! I'm torn between the old characters used for up/down in Utf8WaveRenderer (link) (which match the transitions on binary signals) and the new ones you've chosen (which I find do make it easier to read). Just putting my two cents out there.

[fdxmw]

I'm torn between the old characters used for up/down in Utf8WaveRenderer (link) (which match the transitions on binary signals)

Thanks for taking a look! Can you help me understand how the old up/down characters matched the transitions on binary signals? For me the old characters didn't quite align, see the little bumps and gaps on screenshot, while the new characters seem to align for various fonts I've experimented with (screenshot)

[fdxmw]

Also this is all very subjective, I think these are improvements but I'm happy to make changes if people see things differently :)

[codecov-commenter]

Codecov Report

Patch coverage: 96.03% and project coverage change: -0.04 :warning:

Comparison is base (bec4c37) 90.51% compared to head (a9277d8) 90.47%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #428 +/- ## =============================================== - Coverage 90.51% 90.47% -0.04% =============================================== Files 24 24 Lines 6071 6142 +71 =============================================== + Hits 5495 5557 +62 - Misses 576 585 +9 ``` | [Impacted Files](https://codecov.io/gh/UCSBarchlab/PyRTL/pull/428?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=UCSBarchlab) | Coverage Δ | | |---|---|---| | [pyrtl/passes.py](https://codecov.io/gh/UCSBarchlab/PyRTL/pull/428?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=UCSBarchlab#diff-cHlydGwvcGFzc2VzLnB5) | `94.70% <ø> (+0.04%)` | :arrow_up: | | [pyrtl/simulation.py](https://codecov.io/gh/UCSBarchlab/PyRTL/pull/428?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=UCSBarchlab#diff-cHlydGwvc2ltdWxhdGlvbi5weQ==) | `93.00% <95.96%> (-0.44%)` | :arrow_down: | | [pyrtl/compilesim.py](https://codecov.io/gh/UCSBarchlab/PyRTL/pull/428?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=UCSBarchlab#diff-cHlydGwvY29tcGlsZXNpbS5weQ==) | `91.03% <100.00%> (-0.47%)` | :arrow_down: | ... and [6 files with indirect coverage changes](https://codecov.io/gh/UCSBarchlab/PyRTL/pull/428/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=UCSBarchlab) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=UCSBarchlab). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=UCSBarchlab)

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[mdko]

I'm torn between the old characters used for up/down in Utf8WaveRenderer (link) (which match the transitions on binary signals)

Thanks for taking a look! Can you help me understand how the old up/down characters matched the transitions on binary signals? For me the old characters didn't quite align, see the little bumps and gaps on screenshot, while the new characters seem to align for various fonts I've experimented with (screenshot)

Oh, sorry, I think I was looking at the wrong picture! Your new version looks much better, including those transitions. I’m 100% pro your changes. 😎

[fdxmw]

A number of updates to this pull request:

1. Rising and falling edges are now vertical. This is done by rendering binary wires across two lines in the terminal.
2. Added an environment variable option PYRTL_RENDERER to select a particular renderer. There are currently 5:
   1. 'powerline' : Render value transitions with powerline triangle glyphs. This produces the familiar hexagon shapes that are common in logic analyzers. Binary wires are rendered with vertical rising and falling edges. Unfortunately powerline glyphs are not standard, and there doesn't seem to be a way to detect their availability, so users must manually enable this option.
   2. 'utf-8' : Default renderer on all platforms except Windows. Binary wires are rendered with vertical rising and falling edges, and values are rendered in rectangles.
   3. 'utf-8-alt' : Binary wires are rendered with diagonal rising and falling edges, and values are rendered in rectangles.
   4. 'cp437' : Default renderer on Windows. Binary wires are rendered with vertical rising and falling edges, using only the box drawing characters in 8-bit ASCII.
   5. 'ascii' : Basic renderer, limited to 7-bit ASCII.
3. Removed render_trace()'s segment_delim and extra_line options. extra_line is now mandatory with two-line rendering for binary wires, and segment_delim complicates the code and doesn't seem to be used.
4. Decreased render_trace()'s default segment_size to 1. I often find myself counting cycles between tick marks, and I find it's much easier to just read the cycle number from the ruler.