Loosen type restriction on Ryu.writeshortest

[nickrobinson251]

Ryu.writeshortest is documented to have a method that "allows passing in a byte buffer", just like Ryu.writefixed and Ryu.writeexp, but unlike those functions writeshortest is type constrained to ::Vector{UInt8}. This PR loosens that to ::AbstractVector{UInt8}, to allow the "byte buffer" to e.g. be a Memory rather than a Vector.

I've added tests and updated the docstrings for all three functions to ensure that they're not just restricted to Vector{UInt8}.

This change was prompted by our private codebase hitting MethodError: no method matching writeshortest(::Memory{UInt8}, ::Int64, ::Float64, ...) when trying it out with Julia v1.12.0-DEV.

(cc @quinnj -- i think you added this method originally, but i couldn't see any reason why e.g. Memory shouldn't be allowed now we have it)

[quinnj]

Yeah, great change. This was all pre-Memory, and Vector{UInt8} was really the only low-level efficient blob thing, so this is a good update.

[nickrobinson251]

CI failures look unrelated

[nickrobinson251]

Thanks, Jacob!