Start rustfmt'ing

[pkgw]

Apply the format everywhere, check it in CI, and give a recommended method for keeping things rustfmt'ed going forward.

[codecov]

Codecov Report

Merging #282 into master will increase coverage by 0.3%.
The diff coverage is 45.56%.

@@            Coverage Diff            @@
##           master     #282     +/-   ##
=========================================
+ Coverage   39.59%   39.89%   +0.3%     
=========================================
  Files         135      135             
  Lines       59315    61091   +1776     
=========================================
+ Hits        23483    24371    +888     
- Misses      35832    36720    +888

Impacted Files	Coverage Δ
tests/tex-outputs.rs	97.64% <ø> (+1.12%)	⬆️
src/lib.rs	0% <ø> (ø)	⬆️
src/bin/tectonic.rs	0% <0%> (ø)	⬆️
src/engines/bibtex.rs	0% <0%> (ø)	⬆️
src/io/local_cache.rs	0% <0%> (ø)	⬆️
src/engines/spx2html.rs	0% <0%> (ø)	⬆️
src/io/itarbundle.rs	0% <0%> (ø)	⬆️
src/io/stdstreams.rs	2.56% <0%> (+0.39%)	⬆️
src/io/zipbundle.rs	0% <0%> (ø)	⬆️
xdv/src/lib.rs	0% <0%> (ø)	⬆️
... and 50 more

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