Properly represent Tuples in the TypeNode AST (#8648)

Properly represent Tuples in the TypeNode AST (#8648)

This makes it so that Tuple types are properly represented in the AST. It also removes a spurious restriction that disallowed higher-order tuples (this was leftover from a very old sanity check in the old API).

For example, a tuple type over (Int, Int) is now (TUPLE_TYPE INT INT) instead of a DATATYPE_TYPE constant.

Tuple types behave exactly like datatypes; we can still retrieve their DType as before.

This is in preparation for gradual types and symbolic tuple projections.