Add backend-dependent Int::MAX, Int::MIN, Double::MAX, Double::MIN

[jiribenes]

It would be very nice to have constants like Int::MAX, Int::MIN, Double::MAX, Double::MIN that determine the sizes of numeric types available on the available backend.

So for Int::MAX, we'd get:

JavaScript: Number.MAX_SAFE_INTEGER => 2^53 - 1
LLVM: 2^63 - 1 (directly?)
Chez Scheme: ???

Feel free to bikeshed the exact names: my intuition was to make a namespace Int and a namespace Double and put the constants there?

[jiribenes]

As an alternate design, for my use case, I'd also be fine if we limited these by the "worst" backend so that they're uniform. This means that Int::MAX would return 2^53 - 1 even on LLVM.