Start working on type-level interpreter

[CohenArthur]

No description provided.

[CohenArthur]

• Split off the Kind::UnionType commits in their own PR
• Merge Add fir::Kind::UnionType #640
• Merge Name resolve multi types #636
• Actual does not take care of resolving TypeVariable::Actual(Kind::TypeReference) to TypeVariable::Actual(UnionType) or TypeVariable::Actual(RecordType)
• Rework commit log
• void needs to be a special typeset
• Handle the following piece of code:

type Some(of: int);
type Nothing;
type Maybe = Some | Nothing;

func unit() -> Some | Nothing { Nothing }
func just(value: int) -> Maybe { Some(of: value) }

[CohenArthur]

• void needs to be a special typeset

[CohenArthur]

Closing in favor of #641