added convert(::Type{UUID}, ::ASCIIString)

Actually `convert` [doesn't make a copy either](https://github.com/JuliaLang/julia/blob/06b809c848343402a9a591b65a076ec0a7f9f30a/base/ascii.jl#L101), although it might still be worth avoiding the unnecessary error check.

Also, `::ASCIIString` is a syntax error.