New evaluator architecture
Let us break everything!
Rip out old evaluator method and make way for...

TAGGED POINTERS
Remove distinction between IntVal and FloatVal.

In the evaluator, the main number type will be a NoahNumber.
Factor NoahNumber into own translation unit.

Also pulled out other necessary bits from evaluator.hh to a new header
file called noah.hh
Get Noah to compile cleanly again.
Few more syntactical ideas
Arithmetic in evaluator.

Using the new tagged pointer setup. Have implemented addition,
subtraction and multiplication. Not tested it though. :-/