Add `GenericArgs::as_closure`

`GenericArgs` is missing [`as_closure`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/type.GenericArgs.html#method.as_closure), which is super useful as closures have a quirky encoding into generics that make it particularly annoying to manipulate.

Relatedly, having `DefId::is_closure_like` would be nice!