Rename the function Bitmap::len to Bitmap::bit_len to clarify its meaning

[HaoYang670]

Signed-off-by: remzi 13716567376yh@gmail.com

Which issue does this PR close?

Closes #1233 .

Rationale for this change

The name bit_len is more clear because the function return the length of bits (not bytes).

What changes are included in this PR?

Are there any user-facing changes?

The function name is changed.

[codecov-commenter]

Codecov Report

Merging #1242 (cbdf4c3) into master (731e132) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1242      +/-   ##
==========================================
- Coverage   82.96%   82.95%   -0.01%     
==========================================
  Files         178      178              
  Lines       51522    51522              
==========================================
- Hits        42743    42742       -1     
- Misses       8779     8780       +1     

Impacted Files	Coverage Δ
arrow/src/bitmap.rs	88.13% <100.00%> (ø)
...rquet/src/arrow/record_reader/definition_levels.rs	86.25% <100.00%> (ø)
arrow/src/array/transform/mod.rs	84.64% <0.00%> (-0.13%)	⬇️

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[alamb]

Thanks @HaoYang670