DeclaredType in TypeChecker

[gzanitti]

As part of the Struct integration (mainly typing), we need to improve the way “declared types” are handled so that we can type struct fields without passing new data structures to type checking.

This PR introduces the necessary modification to then add Struct typing on top of it, preventing #1910 from being huge.

[chriseth]

This looks very good now, thanks! Now my main remaining question is: Can we turn the two enums into a single one?

[gzanitti]

This looks very good now, thanks! Now my main remaining question is: Can we turn the two enums into a single one?

Do you mean DeclaredType and DeclaredTypeKind? Maybe adding the type and an Option<HashMap<...>> to DeclaredType? I find the Option more cumbersome than having another Enum, but whatever you prefer :)