Removed Buffer::into_mut and make_mut functions

[jorgecarleitao]

The function get_mut covers both cases, making it the entry point for cow semantics.

[codecov]

Codecov Report

Merging #1089 (939c9f4) into main (acd14f8) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1089      +/-   ##
==========================================
+ Coverage   81.01%   81.02%   +0.01%     
==========================================
  Files         367      367              
  Lines       35340    35328      -12     
==========================================
- Hits        28629    28626       -3     
+ Misses       6711     6702       -9     

Impacted Files	Coverage Δ
src/buffer/bytes.rs	74.13% <ø> (ø)
src/buffer/immutable.rs	88.88% <ø> (+9.87%)	⬆️
src/array/utf8/mutable.rs	81.33% <100.00%> (-0.06%)	⬇️
src/compute/arity_assign.rs	62.50% <100.00%> (-1.50%)	⬇️
src/bitmap/assign_ops.rs	80.64% <0.00%> (-2.42%)	⬇️
src/bitmap/utils/slice_iterator.rs	97.56% <0.00%> (-1.22%)	⬇️
src/io/ipc/read/reader.rs	95.80% <0.00%> (+0.38%)	⬆️
src/array/binary/mod.rs	90.04% <0.00%> (+1.24%)	⬆️

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