implement Trait std::iter::Step for BigInt

[iosmanthus]

Maybe it will be more convenient to write the following code after adding trait Step for BigInt:

for i in BigInt::from(1)..BigInt::from(1000) {
    // do something
}

[cuviper]

std::iter::Step is still unstable, tracked in rust-lang/rust#42168.

That's a big reason why the num-iter crate exists, like its range function. But if your endpoints are small enough to fit in primitive types, like your example, it might actually be faster to just iterate that way and then convert: (1..1000).map(BigInt::from)

[Rudxain]

If anyone reading this needs something like 0.. (RangeFrom trait), here you go:

std::iter::successors(Some(BigUint::zero()), |n| Some(n + 1u8))

I love successors because it's quite simple and flexible. It helped me refactor many loops into functional-style chains of data