Introduce Tuple.element_type and NamedTuple.element_type

[HertzDevil]

These macros act as compile-time equivalents of Enumerable.element_type, and encapsulate the typeof(begin; ...; end) pattern found in e.g. #11834 and #12008.

Due to syntax limitations, the line below will not work:

Tuple(Int32, String).element_type(1) # Error: undefined method 'element_type' for Tuple(Int32, String).class

For some reason it doesn't work even on aliases of Tuple instances, so these macros are private for now; the currently use cases do not require this macro to be public or protected. Also note that Enumerable is a module, so Enumerable.element_type is not inherited by Tuple and NamedTuple.