Use compiler, not introspection, to find nested typedefs

Use the compiler, rather than introspection, to find the type corresponding to T::value_type or T::member_type
for any Wrapper<T>.
In ROOT 6 this avoids unnecessary header autoparsing, and thereby saves memory.  Since there is nothing specific to ROOT 6 about this change, so for code commonality this change is being made in the main 7_3_X branch.
I ran the full relval matrix, and no new errors occurred.