[BREAKING CHANGE] Subtyping based `runtime.Type_Info`

[gingerBill]

Convert the usage of runtime.Type_Info from being a union of the variants in-place to based on subtypes in order to minimize memory usage and initialization.

This is a BREAKING CHANGE for any code base that uses runtime type information.

How to fix your code:

• case runtime.Type_Info_* -> case ^runtime.Type_Info_*
• x.(runtime.Type_Info_*) -> x.(^runtime.Type_Info_*)
• info_for_struct.flags -> info_for_struct.struct_flags