45: Support if- and while-let chains r=matklad a=ChayimFriedman2

RFC 2497 https://github.com/rust-lang/rfcs/blob/master/text/2497-if-let-chains.md.

I'm not sure if this should be a major or minor version bump: it breaks users of rust.ungram but not general users of the crate.

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