Methods for splitting integers into their halves

[scottmcm]

@abgros

To give a real-world example, the fastrand crate has a function which looks like:

fn mul_high_u128(a: u128, b: u128) -> u128 {

Well, the solution to that is not to stabilize lo/high methods, but to make fastrand not need to implement multiplication like that itself.

That crate just wants https://doc.rust-lang.org/std/primitive.u128.html#method.widening_mul to be stable, so it can use a.widening_mul(b).1 instead: https://rust.godbolt.org/z/anbYahYc1

cc Tracking Issue for bigint helper methods · Issue #85532 · rust-lang/rust · GitHub