Optimize passing multiple paths to usort CLI

[amyreese]

Previously, when passing multiple paths to the usort CLI, it would
call usort_path() separately for each path given, preventing any
chance of parallel processing. This means usort check usort/**/*.py
would take ~10X longer than usort check usort, and also results in
double formatting if a path appeared or was a child of multiple
arguments:

(.venv) jreese@butterfree ~/workspace/usort main  » time usort check usort
usort check usort  3.64s user 0.32s system 338% cpu 1.169 total

(.venv) jreese@butterfree ~/workspace/usort main  » time usort check usort usort usort
usort check usort usort usort  10.67s user 0.89s system 470% cpu 2.458 total

(.venv) jreese@butterfree ~/workspace/usort main  » time usort check usort/**/*.py usort/**/*.py usort/**/*.py
usort check usort/**/*.py usort/**/*.py usort/**/*.py  13.41s user 1.59s system 62% cpu 24.141 total

With the changes from #160, this is partially masked by removing the
cost of starting child processes, but still results in linear time
increase when passing many arguments:

(.venv) jreese@butterfree ~/workspace/usort patch-1  » time usort check usort
usort check usort  3.68s user 0.36s system 318% cpu 1.267 total

(.venv) jreese@butterfree ~/workspace/usort patch-1  » time usort check usort usort usort
usort check usort usort usort  10.37s user 0.86s system 483% cpu 2.323 total

(.venv) jreese@butterfree ~/workspace/usort patch-1  » time usort check usort/**/*.py usort/**/*.py usort/**/*.py
usort check usort/**/*.py usort/**/*.py usort/**/*.py  2.45s user 0.06s system 89% cpu 2.803 total

This builds on #160 by updating usort_path() to optionally take an
iterable of Path objects, and materializes a set of paths to sort before
actually starting child processes. This allows maximum utilization of
multiprocessing, and also eliminates any double-formatting if the CLI
arguments include any overlap of paths:

(.venv) jreese@butterfree ~/workspace/usort gather  ‹1› » time usort check usort
usort check usort  3.82s user 0.36s system 333% cpu 1.255 total

(.venv) jreese@butterfree ~/workspace/usort gather  » time usort check usort usort usort
usort check usort usort usort  3.88s user 0.37s system 356% cpu 1.192 total

(.venv) jreese@butterfree ~/workspace/usort gather  » time usort check usort/**/*.py usort/**/*.py usort/**/*.py
usort check usort/**/*.py usort/**/*.py usort/**/*.py  3.68s user 0.36s system 323% cpu 1.249 total