[META] opaque type handling

[lcnr]

Opaque types are have some non-trivial backcompat requirements and generally require a lot of care to support correctly.

misc

• https://github.com/rust-lang/trait-system-refactor-initiative/issues/29
• https://github.com/rust-lang/trait-system-refactor-initiative/issues/32
• https://github.com/rust-lang/trait-system-refactor-initiative/issues/49

problematic may-define outside of typeck

• 106

normalizing opaque types allows more code to compile

• https://github.com/rust-lang/trait-system-refactor-initiative/issues/112

treating opaque as rigid during typeck

• https://github.com/rust-lang/trait-system-refactor-initiative/issues/131