ARROW-11034: [Rust] remove rustfmt ignore list, fix format

[mqy]

In this PR:

• Ignore list is commented out from rustfmt.toml, thus carrgo +stable fmt would not show warnings
• Two files are re-formatted by nightly, stable: this may avoid formatting problem we had seen in CI (hope so).

Shell commands that I had run:

cargo +nightly-2020-11-24-x86_64 fmt
cargo +1.48.0-x86_64 fmt

[codecov-io]

Codecov Report

Merging #9013 (4e40312) into master (a4f7c4a) will decrease coverage by 0.00%.
The diff coverage is 68.42%.

@@            Coverage Diff             @@
##           master    #9013      +/-   ##
==========================================
- Coverage   82.87%   82.87%   -0.01%     
==========================================
  Files         201      201              
  Lines       49739    49743       +4     
==========================================
+ Hits        41220    41223       +3     
- Misses       8519     8520       +1     

Impacted Files	Coverage Δ
rust/parquet/src/record/api.rs	97.88% <66.66%> (-0.13%)	⬇️
rust/datafusion/src/scalar.rs	59.07% <68.75%> (-0.08%)	⬇️
rust/parquet/src/encodings/encoding.rs	95.43% <0.00%> (+0.19%)	⬆️

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[github-actions]

https://issues.apache.org/jira/browse/ARROW-11034

[alamb]

Looks good to me

I checked this branch out locally and ran these two commands and there were no changes to my local workspace. 🎉

cargo +stable fmt
cargo +nightly-2020-11-24 fmt

[alamb]

Thanks @mqy !