ARROW-2691: [Rust] Update code formatting with latest Rust stable

[andygrove]

No description provided.

[codecov-io]

Codecov Report

Merging #2126 into master will increase coverage by 0.95%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2126      +/-   ##
==========================================
+ Coverage   86.37%   87.33%   +0.95%     
==========================================
  Files         230       12     -218     
  Lines       40589      892   -39697     
==========================================
- Hits        35059      779   -34280     
+ Misses       5530      113    -5417

Impacted Files	Coverage Δ
rust/src/array.rs	84.95% <ø> (ø)
cpp/src/arrow/ipc/writer.h
cpp/src/arrow/array.h
cpp/src/arrow/python/util/datetime.h
cpp/src/arrow/util/cpu-info.cc
cpp/src/arrow/ipc/message.h
cpp/src/arrow/io/io-buffered-test.cc
python/pyarrow/tests/test_deprecations.py
cpp/src/plasma/plasma.cc
cpp/src/arrow/python/numpy-internal.h
... and 233 more

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[wesm]

+1, thanks Andy. It seems like you might need to git reset --hard upstream/master on your master branch to get rid of all those merge commits

[xhochy]

+1, LGTM

Thanks. In the longterm, should we somehow pin the version of rustfmt as we do with clang-format?