Renamed Bitmap::null_count to Bitmap::unset_bits

[jorgecarleitao]

null_count is not a very relatable name to a "bitmap", since the association null <> unset_bit only emerges in arrays - this PR renames the function (and marks the old as deprecated).

[codecov]

Codecov Report

Merging #1087 (f1a0bcc) into main (d1ab4ef) will increase coverage by 0.02%.
The diff coverage is 91.22%.

@@            Coverage Diff             @@
##             main    #1087      +/-   ##
==========================================
+ Coverage   81.07%   81.10%   +0.02%     
==========================================
  Files         367      367              
  Lines       35309    35315       +6     
==========================================
+ Hits        28628    28641      +13     
+ Misses       6681     6674       -7     

Impacted Files	Coverage Δ
src/io/parquet/write/binary/basic.rs	68.14% <0.00%> (ø)
src/bitmap/mutable.rs	96.86% <40.00%> (-0.66%)	⬇️
src/bitmap/immutable.rs	81.48% <90.00%> (-1.54%)	⬇️
src/array/binary/mutable.rs	87.97% <100.00%> (ø)
src/array/boolean/mutable.rs	89.32% <100.00%> (ø)
src/array/mod.rs	68.54% <100.00%> (ø)
src/array/physical_binary.rs	97.67% <100.00%> (ø)
src/array/primitive/mutable.rs	95.57% <100.00%> (ø)
src/array/utf8/mutable.rs	84.00% <100.00%> (ø)
src/bitmap/assign_ops.rs	83.06% <100.00%> (ø)
... and 15 more

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