Add Compat.readuntil

[carlobaldassi]

This is to allow for the keep keyword argument, which was added in JuliaLang/julia#25646 (while also setting it to false by default - this was a breaking change).

The code is basically copy-pasted from current Base with a few back-porting changes.
The tests are also taken from the Julia repo, basically as-is.

[stevengj]

Wouldn't it be simpler to just define something like:

stripdelim(s, d::Union{Char,UInt8}) = s[end] == Char(d) ? s[1:prevind(s,lastindex(s))] : s
stripdelim(s, d::AbstractString) = endswith(s, d) ? s[1:prevind(s,lastindex(s),length(d)] : s
function readuntil(f, d; keep::Bool = false)
    s = Base.readuntil(f, d)
    if keep || isempty(s)
        return s
    else
        return stripdelim(s, d)
    end
end

This is a bit slower, of course, but optimizing speed on 0.6 is not a big priority for Compat.

[carlobaldassi]

Ok I used the slower simpler version, keeping the tests from Base.

[stevengj]

You can use prevind(s, i, n) as I originally suggested in my comment, since this was added to Compat in #573.

[carlobaldassi]

Ah, I didn't have the latest master. Fixed, thanks.

[stevengj]

String(d) seems wrong here. Maybe String(reinterpret(UInt8, d))?

[stevengj]

The convert(T, also seems wrong, since String doesn't have a convert method for an arbitrary array type

in general, it looks like this will fail for T != UInt8 here?

[stevengj]

Maybe we can just support d::Vector{UInt8} in Compat, since I'm guessing few people need other vector types.

[carlobaldassi]

I have updated the code now (sorry for the delay, broken laptop...).
@stevengj I have restricted support to Vector{UInt8} and Vector{Char}, which seem to be the two relevant cases.