Add `SizeTrackingChannel`

[chakaz]

This type of channel is very similar to SimpleChannel, but it also tracks the size of Ts it owns.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (57e34ef) 77.45% compared to head (e05641c) 77.45%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #170 +/- ## ======================================= Coverage 77.45% 77.45% ======================================= Files 98 98 Lines 7221 7221 ======================================= Hits 5593 5593 Misses 1628 1628 ``` [see 2 files with indirect coverage changes](https://app.codecov.io/gh/romange/helio/pull/170/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman)

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[chakaz]

queue capacity * size of object is how much memory is used by the queue, am I missing something?

[chakaz]

I see what you may mean, I'll need to track each element's size as we save a string there as well :|

[romange]

If this object a string or any other object that uses heap then then the important bit is missing. We do not care about 'sizeof string * capacity'