That was always the intention, but the cleanup code was always
placed outside the unwind forms.

  lib/rust-mode/rust-rustfmt.el:60:12: Warning: ‘unwind-protect’
    without unwind forms

"rust-prog-mode.el" and "rust-mode-treesitter.el" provide competing
implementations of `rust-mode'.  Both implementations depend on code
in "rust-mode.el", and thus must require that.

Doing that is complicated by the fact that "rust-mode.el" loads one
of these libraries, depending on `rust-mode-treesitter-derive's value.

Address this conflict by:

1. Requiring feature `rust-mode' in the two libraries that implement the
   `rust-mode' major-mode and that use things defined in "rust-mode.el".

2. Moving the require forms for these two libraries in "rust-mode.el",
   below the `provide' form for `rust-mode'.