Gleam: Refactor ClauseGuard to only have a type field for Var

[lpil]

https://github.com/gleam-lang/gleam/blob/113c89fc15753d5f7f4efbec3f65af253a64a07e/src/ast.rs#L280-L287

Currently we store a Type value on all nodes, but the value contained only varies on the Var node. Remove this field from all other nodes and update the typ method to return a constant value rather than the contained field.

[tomwhatmore]

I can do this unless you'd rather people only did one good first issue?

[lpil]

Go for it Tom!

[CrowdHailer]

Is there anything remaining on this issue?

[lpil]

Well spotted, thanks Peter!