BigInt ranges

[bamarsha]

From microsoft/qsharp-language#132:

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);
    ...
}

[sezna]

It could make sense to approach this issue by implementing generic UDTs, making ranges generic, and then re-implementing Int ranges, as well as BigInt ranges, in that new Range type.

[nirajvenkat]

Want to nudge this issue as it was mentioned in the call yesterday.