implement String(::Base.Generator)

[StefanKarpinski]

I was hoping that this would work:

String(Iterators.map(c->c+1, "Hello, world"))

Instead it's a method error. We should allow the String constructor to take a generator argument.

[ararslan]

Perhaps a method that consumes general iterators would be worthwhile, e.g.

🍝(x, ::Union{HasLength,HasShape}) = String(copyto!(StringMemory(length(x)), x))
🍝(x, ::IsInfinite) = throw(MethodError(String, (x,)))
function 🍝(x, ::SizeUnknown)
    io = IOBuffer()
    for c in x
        print(io, convert(AbstractChar, c))
    end
    return String(take!(io))
end

String(itr) = 🍝(itr, IteratorSize(itr))

That would allow supporting things like String(Iterators.take("Hello, world", 5)) as well.