cleanup(fs): stricter check for buffer chunks count and minor improvements

[kskalski]

Problem

u16 is used for index of buffer chunks (IoBufferChunk), e.g. in FileState there is start_buf_index: Option<u16>,
However we don't validate that dividing original buffer into chunks won't exceed limit of u16.

Summary of Changes

• assert that dividing buffer.len() by chunk_size doesn't exceed u16::MAX (also add check that chunk_size is not larger than FIXED_BUFFER_LEN)
• switch some u32 types in function signatures to proper type alias IoSize
• add some safety comments
• minor cleanups

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.6%. Comparing base (1e0f899) to head (c119ed2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10117   +/-   ##
=======================================
  Coverage    82.6%    82.6%           
=======================================
  Files         845      845           
  Lines      317132   317134    +2     
=======================================
+ Hits       261967   261982   +15     
+ Misses      55165    55152   -13     

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[kskalski]

this is now compatible with the way buf_start is calculated in line 211

[kskalski]

this was supposed to be part of #9701, missed pushing changes from my local repo.. :/

[brooksprumo]