should implement `readbytes`

[JeffBezanson]

I hereby apologize for the readall function. I think GZip should implement readbytes instead, and inherit readall(::IO) that just wraps the result of readbytes in a string.

[quinnj]

Yeah, it should really implement readbytes! or even better would be readavailable; as-is, the only standard IO function implemented is read(::GZipStream, ::Type{UInt8}), which means the majority of cases where a GZipStream is used generically falls back to byte-by-byte IO, which is probably terrible for performance.