Add tuples to the language

[Victorious3]

Currently the parser only knows quasi tuple syntax for function returns:

def function -> int, int

This is already implemented using a Tuple type. It should be trivial to allow the use of tuples outside of function context.

var t1: (int, int) = (10, 20)
var t2: (int,) = (10,)    // Single elements like python

The parser already does build a list of types for function types, the way to go would be to check if the next token is an -> and pick a function type in that case and a tuple type in the other.

[Victorious3]

Related to #36