Mark MutableBuffer::typed_data_mut unsafe

[alamb]

Which issue does this PR close?

Closes #1027

Rationale for this change

Make the rust type annotations match the docstring about safety, and slightly decreasing the chance of misusing the apis in this crate

What changes are included in this PR?

1. Mark MutableBuffer::typed_data_mut unsafe
2. Annotate calls to MutableBuffer::typed_data_mut as unsafe and add justification

Are there any user-facing changes?

See subject

[alamb]

FYI @jorgecarleitao and @jhorstmann

[codecov-commenter]

Codecov Report

Merging #1029 (f1f32a6) into master (e0abda2) will decrease coverage by 0.00%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #1029      +/-   ##
==========================================
- Coverage   82.31%   82.31%   -0.01%     
==========================================
  Files         168      168              
  Lines       49031    49031              
==========================================
- Hits        40359    40358       -1     
- Misses       8672     8673       +1     

Impacted Files	Coverage Δ
arrow/src/buffer/mutable.rs	89.47% <75.00%> (ø)
arrow/src/buffer/ops.rs	96.77% <100.00%> (ø)
arrow/src/compute/kernels/sort.rs	94.95% <100.00%> (ø)
arrow/src/compute/kernels/take.rs	95.21% <100.00%> (ø)
parquet/src/arrow/array_reader.rs	76.75% <100.00%> (ø)
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.46%)	⬇️
arrow/src/datatypes/datatype.rs	66.38% <0.00%> (+0.42%)	⬆️

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