Add Context.storeType

I'd like something like `Context.storeTypedExpr` for types.

In tink_macro this behavior is emulated per `@:genericBuild class DirectType<@const Id> {}` where the type is stored with a numerical id and then looked up again by id in the generic build macro.

I would very much like for this to be part of the compiler though, especially since calls like `TypeTools.toComplexType` could use it to represent everything (ideally it'd still try to create a proper `ComplexType`, but it could fill the unrepresentable blanks with something better than `null`).