Support rustfmt checks

[jrobsonchase]

Might be nice to have some out-of-the-box support for format checking. I'm doing so in a testPostRun in some personal projects at the moment:

testPostRun = ''
  if [ "$_FMT_CHECKED" != "1" ]; then
    PATH=${toolchain}/bin:$PATH
    cp ${cargonix}/crate/rustfmt.toml .
    echo "Checking formatting..."
    _checkFiles=$(find . -name '*.rs')
    echo -e "$_checkFiles\n"
    rustfmt --check $_checkFiles
    _FMT_CHECKED=1
  fi
'';

Doing it in crate2nix proper might help with the cargonix and toolchain plumbing, as well as remove the need for the guard to prevent checking multiple times.