Fixed clippy and fmt

[ritchie46]

Rebased on top of #519

Fixed most clippy lints. The remaining are unsafe docs that miss a # Safety tag. As I don't know all invariants, I left this open for you @jorgecarleitao.

There is also a lint for a field that's not used. Removing it requires breaking changes, so I want to discuss this first.

warning: field is never read: `is_optional`
   --> src/io/parquet/write/levels.rs:151:5
    |
151 |     is_optional: bool,
    |     ^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default

[codecov]

Codecov Report

Merging #521 (577081c) into main (e77d4e1) will decrease coverage by 0.01%.
The diff coverage is 87.80%.

@@            Coverage Diff             @@
##             main     #521      +/-   ##
==========================================
- Coverage   80.18%   80.16%   -0.02%     
==========================================
  Files         381      381              
  Lines       23665    23662       -3     
==========================================
- Hits        18975    18969       -6     
- Misses       4690     4693       +3     

Impacted Files	Coverage Δ
src/array/display.rs	61.15% <0.00%> (ø)
src/array/ffi.rs	65.00% <ø> (ø)
src/compute/cast/mod.rs	88.83% <ø> (+0.15%)	⬆️
src/io/parquet/read/statistics/fixlen.rs	39.53% <0.00%> (-4.66%)	⬇️
src/io/parquet/write/schema.rs	63.13% <ø> (+0.28%)	⬆️
src/scalar/primitive.rs	81.48% <ø> (ø)
src/array/binary/mutable.rs	65.08% <100.00%> (ø)
src/array/utf8/ffi.rs	78.94% <100.00%> (+1.16%)	⬆️
src/array/utf8/mutable.rs	85.25% <100.00%> (ø)
src/bitmap/bitmap_ops.rs	93.81% <100.00%> (ø)
... and 16 more

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