remove repr(::UUID), move UUID code

[ivarne]

Looking at #17017, I noticed that there is room for more specialized (eg. faster) method specializations for repr().

julia> methods(repr)
#2 methods for generic function "repr"
repr(u::Base.Random.UUID) at random.jl:1252
repr(x) at strings/io.jl:86

The generic implementation in strings/io.jl:85 uses an IOBuffer, so it is probably much slower than it needs to be (especially for Strings which I think could be implemented as repr(s::string) = s). The documentation demands the result must be the same as showall, so testing should ensure that is still the case.

[vtjnash]

Since this function is just an alias for showall, I propose we eliminate it.

[nalimilan]

More precisely, it's an alias for sprint(showall, ...), right? I agree about deprecating it, since in 0.5 what's often needed is sprint(showcompact, ...) instead, and the fact that we have repr can cause confusion.

[JeffBezanson]

It is not the case that repr(x::String) = x, since repr adds quotes and escaping.

[ivarne]

@JeffBezanson Thanks for noticing. I somehow managed to convince myself that it did not, but my REPL history shows I did not actually try.

[JeffBezanson]

If anything, the repr method for UUID should be removed since nothing else defines it directly. Instead it should define string or show. Then the UUID code is a good candidate for #5155, since AFAICT it's not used anywhere in Base.

[tkelman]

Some of the UUID code will be needed for Pkg3, but if that itself is a package then it can have other-package dependencies that aren't all Base code.

[StefanKarpinski]

Making this a method of string would be the way to go here. Moving UUIDs out would be good too.