Made len/len_proxy consistent with Offsets

[ritchie46]

We already renamed this in Offsets because it returned a counterintuitive len. This bit me again. ;)

This makes sure we are consistent with Offsets and OffsetsBuffer.

[codecov]

Codecov Report

Patch coverage: 68.42% and project coverage change: -0.01 ⚠️

Comparison is base (db87f71) 83.76% compared to head (adf4571) 83.76%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1434      +/-   ##
==========================================
- Coverage   83.76%   83.76%   -0.01%     
==========================================
  Files         375      375              
  Lines       41024    41027       +3     
==========================================
+ Hits        34364    34366       +2     
- Misses       6660     6661       +1     

Impacted Files	Coverage Δ
src/compute/aggregate/memory.rs	37.50% <0.00%> (ø)
src/offset.rs	84.87% <40.00%> (-0.89%)	⬇️
src/array/binary/mod.rs	91.82% <100.00%> (+1.44%)	⬆️
src/array/list/mod.rs	91.74% <100.00%> (ø)
src/array/map/mod.rs	62.13% <100.00%> (ø)
src/array/utf8/mod.rs	82.37% <100.00%> (ø)
src/io/parquet/write/pages.rs	95.84% <100.00%> (ø)

... and 3 files with indirect coverage changes

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