Add tuple support

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This is a PR to add tuple support to Coalton. Refer to #15. Here are the TODOs:

• Add type syntax for tuples (parsing and unparsing). Proposal: (* t1 t2 ... tn)
• Allow tuples to be type checked and unified.
• Decide how tuple values are compiled into Lisp. Proposal: simple-vector
• Add syntax for tuple construction. Proposal: not sure
• Add depth-1 pattern matching.

I'm not sure about this one, but...

• Add syntax for tuple projection. (Note that this can't be a generic function taking a generic integer argument. The syntax would have to be special, like (@ <integer> <expr>) e.g. (@ 15 v) which verifies that the value v is a 15-or-fewer tuple.) This might be hard to type check.