Reflection defines two extensions named `memberType` with different meaning

[smarter]

It's very confusing that:

• tp.memberType(sym) gives you back the type of the member sym as seen from tp
• but sym.memberType(name) gives you back the symbol of a type member named name

IMO, we should rename the method on symbol to typeMember instead (same with a bunch of other methods on member like memberField, etc).