FullParameterization

Assuming info is a result of a fullyParameterizedType call, the signature of the original method type X after stripping its leading type parameters section, such that: info.stripPoly.ensureMethodic = fullyParameterizedType(X, ...).stripPoly.ensureMethodic

NOTE: Keeping the polymorphic part of the signature would be more precise, but we cannot distinguish which type parameters of info are also type parameters ofX. This could be fixed by using a specific NameKind for the extra type parameters, but that wouldn't help for extension methods unpickled from Scala 2 (because Scala 2 extmeths phase happens before pickling, which is maybe something we should change for 2.14).