feat(IoBuf): Allow getting and accumulating memory stats

[codecov-commenter]

Codecov Report

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

Comparison is base (1b3f13d) 77.41% compared to head (90e7b55) 77.49%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #167 +/- ## ========================================== + Coverage 77.41% 77.49% +0.08% ========================================== Files 98 98 Lines 7217 7217 ========================================== + Hits 5587 5593 +6 + Misses 1630 1624 -6 ``` | [Files](https://app.codecov.io/gh/romange/helio/pull/167?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman) | Coverage Δ | | |---|---|---| | [base/io\_buf.h](https://app.codecov.io/gh/romange/helio/pull/167?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-YmFzZS9pb19idWYuaA==) | `90.00% <100.00%> (ø)` | | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/romange/helio/pull/167/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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