rust-mode cleanup.

* Use `setq-local' instead of (set (make-local-variable ...) value).
  Provides a version for older Emacsen.
* Remove use of `cl.el'.
* Use \' in file regexp instead of line end match $.
* Use type for defcustom and add parent group.
auto merge of #13432 : ruediger/rust/rustmode, r=nikomatsakis

* Use `setq-local` instead of `(set (make-local-variable 'var) value)`.  Provides a version for older Emacsen.
* Remove use of `cl.el`.
* Use \' in file regexp instead of line end match $.
* Use type for `defcustom` and add parent group.