A better return_type_tinfo() mechanism.

A better return_type_tinfo() mechanism.

return_type_tinfo() is used in order to distingish between non-commutative
objects of different type. However, often it's necessary to distingish
between non-commutative objects of the same type, for example, between
gamma matrices with different representation label. return_type_tinfo()
does not provide any clean way to do that. Hence, introduce return_type_t
type which holds representation label along with type information, and
use it for return_type_tinfo().