Add support for BigInt ranges

[Strilanc]

I'd like to be able to write this:

for k in 0L..15L {
    ...
}

Currently this causes a compile error:

error QS0001: The type Int does not match the type BigInt

It should instead be equivalent to:

for k_helper in 0..15 {
    let k = IntAsBigInt(k_helper);
    ...
}