Merge #332

332: Struct types r=matklad a=flodiebold

Infer types for struct fields, and add basic field completions. There's also some code for enums, but I focused on getting structs working.

There's still ways to go before this becomes useful: There's no autoderef (or even reference types) and no inference for `self`, for example.

Co-authored-by: Florian Diebold <flodiebold@gmail.com>