perf: Add a VarInt encoding for the row encoding

[coastalwhite]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 9.63855% with 150 lines in your changes missing coverage. Please review.

Project coverage is 79.42%. Comparing base (414d883) to head (543e420).

Files with missing lines	Patch %	Lines
crates/polars-row/src/variable/varint.rs	0.00%	132 Missing ⚠️
crates/polars-row/src/encode.rs	47.05%	18 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #19929       +/-   ##
===========================================
+ Coverage   59.28%   79.42%   +20.14%     
===========================================
  Files        1555     1556        +1     
  Lines      216180   216333      +153     
  Branches     2456     2456               
===========================================
+ Hits       128155   171830    +43675     
+ Misses      87467    43945    -43522     
  Partials      558      558               

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