Rust importer is ready for testing

[Edward K. Ream]

This is #1583. It will be included in Leo 6.2 b1.

I have been able to import RustPython's compiler  files with this script:

dir_ = r'C:\leo.repo\RustPython\compiler\src'
c.recursiveImport(
    dir_=dir_,
    kind = '@clean',
    add_path = True,
    recursive = False,
    safe_at_file = True,
    theTypes = ['.rs',]
)

This is a substantial test. leo/plugins/importers/rust.py now handles lots of rust syntax.

The code has been merged from the rust branch into devel and 6.2.

Please report any problems.

Edward