Big Integers

[booniepepper]

This is a scripting language, and 64 bits doesn't make a ton of sense. (Weird to overflow in a scripting language, also weird and unexpected on 32 bit systems!)

Move DtVal.int to std.math.big.int.Mutable

The implementation I'm leaning towards is something like:

const Integer = union(enum) {
    usize: usize,
    big: std.math.big.int.Mutable,

    // Functions for add/sub/mul/div/comparison/etc that take other Integer
    // Functions like isUsize()/toUsize() and isBig()/toBig()
};

Start as usize and coerce up into larger representations. Probably exactly one GPA should be used and shared among all Integer.