ARROW-3969: [Rust] Format using stable rustfmt

[andygrove]

No description provided.

[codecov-io]

Codecov Report

Merging #3138 into master will increase coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3138      +/-   ##
==========================================
+ Coverage   86.87%   87.08%   +0.21%     
==========================================
  Files         492      492              
  Lines       69148    69160      +12     
==========================================
+ Hits        60070    60231     +161     
+ Misses       8977     8832     -145     
+ Partials      101       97       -4

Impacted Files	Coverage Δ
go/arrow/math/uint64_sse4_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/math/float64_sse4_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/memory/memory_sse4_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/math/int64_sse4_amd64.go	0% <0%> (-100%)	⬇️
python/pyarrow/lib.pxd	0% <0%> (ø)	⬆️
go/arrow/math/float64_amd64.go	33.33% <0%> (ø)	⬆️
go/arrow/math/int64_amd64.go	33.33% <0%> (ø)	⬆️
go/arrow/math/uint64_amd64.go	33.33% <0%> (ø)	⬆️
cpp/src/arrow/python/common.h	100% <0%> (ø)	⬆️
python/pyarrow/parquet.py	93.89% <0%> (+0.19%)	⬆️
... and 26 more

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[wesm]

+1