Improve basic conformance and add more unit tests

[behnam]

Improve basic conformance by fixing steps X10, N1/N2, L1

Continue my review of code based on spec and fix various steps that drop the basic conformance failures from 12827 to 250.

• X10: Fix incorrect index for succ_level resolution (off by end_of_seq because of sub-sequencing) and add unit tests from the spec text, from both BD13 and X10 sections.

• N1/N2: Fix resetting rule for NI levels by definint e, as noted in the spec.

• L1: Fix not resetting leading whitespace/formatting chars in a line.

[tests/conformance] Use non-ASCII sample chars

• Use non-ASCII sample chars for gen_char_from_bidi_class(), which increases the failure rate from 250 to 314.

[lib] Impl BidiInfo.reordered_levels_per_char()

• This allows easier testing of the logic, specially having specific cases from the conformance test as unit test, as corner bugs unfold.

[lib] reorder_line(): Expand LTR-only shortcut condition

• There's almost no extra cost in expanding the shortcut condition to check all the isolating run sequences to be LTR, if there's more than one. This can prevent a bunch of string manipulation when all isolating run sequences are LTR.

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This change is 

[behnam]

Done.

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Review status: 0 of 7 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[mbrubeck]

@bors-servo r+

[bors-servo]

:pushpin: Commit bacbb67 has been approved by mbrubeck

[bors-servo]

:hourglass: Testing commit bacbb67c4c7aec66542429606f7c80c118807f3c with merge c2603e2ae37825f820eb227391ce7ce7c752855a...

[bors-servo]

:sunny: Test successful - status-travis Approved by: mbrubeck Pushing c2603e2ae37825f820eb227391ce7ce7c752855a to master...