Merge #2200

2200: Add variables to HIR r=matklad a=matklad

Introduce a `hir::Variable`, which should cover locals, parameters and `self`. Unlike `PatId`, variable knows it's owner so it is self-contained, and should be more convenient to use from `ra_ide_api`.

The goal here is to hide more details about `Body` from hir, which should make it easier to move `Body` into `hir_def`. I don't think that `ra_ide_api` intrracts with bodies directly at the moment anyway, but the glue layer is based basically on `ast::BindPat`, which seems pretty brittle.

Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>