The ambiguous associated types error sometimes fires in cases where it
shouldn't be ambiguous ([1]), which can make things clunky when working
with chained associated types (e.g. `Op::FTy::Int::*` does not work).
Add helper types that we can use instead of the full syntax.
There aren't too many cases in-crate now but this is relevant for some
open PRs.
[1]: https://github.com/rust-lang/rust/issues/38078