Make getbinary return ByteString

[Xandaros]

I also removed the unsafeInterleaveIO part because I don't know what it does. Seems to work without it.

If it does something crucial which I'm missing, you'll have to add it yourself, since I don't know how to do that with ByteStrings.

[jgoerzen]

I think the one feedback I would add is to add a new function that returns a ByteString, rather than remove the existing one, such that we don't introduce an API incompatibility.

[fizruk]

I would rather just bump major version. ByteString is the way to go and String arguably should almost never be used 😄