Add slice::MutableCloneableVector to the prelude

Every other trait in slice is in the prelude, so it makes sense to
provide MutableCloneableVector as well.
Update rust.vim for the latest prelude/keywords
vim: Handle box expressions specially

Attempt to highlight the placement expression in a `box (expr) foo`
expression. Also treat GC as a keyword within the placement expression.

This doesn't work correctly for arbitrary expressions. Notably, this
makes no attempt at balancing delimiters. I believe handling that will
require rewriting the syntax rules to add a region for every pair of
delimiters. That may be a desirable thing to do in the end, because we
can then rewrite our indent rules based on the syntax and get rid of
cindent(), but for the time being, we'll just live with the limitation.
auto merge of #14307 : kballard/rust/vim_prelude_mutablecloneablevector, r=cmr

Add slice::MutableCloneableVector to the prelude. It's the only slice
trait that's currently missing.

Update rust.vim to match the latest prelude and current set of keywords.
Also teach it to handle box placement expressions specially.