47: Support destructuring assignments (RFC 2909) r=Veykril a=ChayimFriedman2

The supported patterns are already valid as expressions, except the rest pattern (`..`) and the wildcard pattern (`_`).

(We really should separate `rust.ungram` into the rust-analyzer repository, it blocks every syntax change).

Co-authored-by: Chayim Refael Friedman <chayimfr@gmail.com>