Feature request: add an AbstractBitSigned type

[rfourquet]

The motivation is increasing re-usability of Base code, in particular from the BitIntegers package.
A bunch of functions are defined for a limited set of types, like Base.BitSigned, but their code would apply as well for any signed "bits" types, so it was copy-pasted in BitIntegers.jl. It would be great to have a type hierarchy like

abstract type AbstractBitSigned <: Signed end
abstract type AbstractBitUnsigned <: Unsigned end
const AbstractBitInteger = Union{AbstractBitSigned,AbstractBitUnsigned}

And then define more functions against these types instead of the closed lists in Base.BitSigned et al.
Re-using the Base.BitSigned name to make it an abstract type instead of AbstractBitSigned would also work, but I don't know how much code this would break.

Making this happen is probably not a simple PR for me as it involves touching C code, hence only an issue (for now at least).

[rfourquet]

Bump. If this would be OK in principle, I might still try to make a PR.

[rfourquet]

Bump.

[nsajko]

Naively this seems like a good idea, because code reuse is good. However it also may be dangerous if not done with a lot of care. What I'm getting at is that some methods may be restricted to hardcoded Unions like BitSigned for safety. For example, if a method has an @assume_effects annotation, or similar, it likely must be restricted to a hardcoded union to avoid UB. So any PR will have to be very carefully written and reviewed, I think.

Defining the semantics of AbstractBitInteger, AbstractBitSigned, etc., in the documentation, also seems potentially challenging.

In any case I think your best bet is to make a PR, a concrete PR is easier to evaluate than a vague issue.