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

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?