perf: use bitwise operations in add/sub impls

[DaniPopes]

See rust-lang/rust#85532 (comment)

[prestwich]

CI failure caused by this:
paritytech/parity-scale-codec#662

[prestwich]

gonna hold this open a couple days to see if parity will patch

[recmo]

Is there a perf difference between using | and || on a bool?

[recmo]

Ah, looks like there is some issue. Rustc has a similar patch: rust-lang/rust@9836196

rust-lang/rust#85532 (comment)

Generally these functions should mirror the bigint helpers from rustc untill they stabelize.

[prestwich]

parity has yanked [email protected] and CI is passing so this is good to go

[scottmcm]

Is there a perf difference between using | and || on a bool?

a | b is BitOr(a, b) internally, whereas a || b is if a { true } else { b } as control-flow, not an operator.

Ideally there wouldn't be a difference, but optimizers are imperfect. In places where you're μoptimizing and would prefer eager behaviour, | can help encourage that.