Built-in product/tuple types and unit types

[nsbgn]

It is possible to have a user-defined Tuple type that allows types like Map(Tuple(A, B), C). This is useful, because CCT types like R3a(X, Y, Z) are more intuitively expressed as Map(X, Tuple(Y, Z)). The downside is that every n-tuple would have to be explicitly defined, and a shorter notation (ie (Y, Z)) would not be possible. A built-in tuple type (to go along with the Function type) would make the library more user-friendly.

[nsbgn]

Alternatively, overloading * to refer to the tuple type, as in Map(X, Y * Z), might be even better. It mirrors the Cartesian product symbol ×, which is the natural symbol for tuple types in mathematics (if not in programming languages), there's fewer parentheses, and it's easier to implement (since adding * for longer tuples will just make nested tuples).

The downside is that it might be confusing with ** being the function arrow, and parsing needing to take this into account. Also, implicitly creating nested tuples is undesirable when you actually need explicitly nested tuples. It is possible to not create nested tuples but add to an existing tuple instead, but then how do you create nested tuples?

[nsbgn]

Product types are now parsed; this issue is otherwise subsumed by #79.