Start stubbing in new IO bits.
Fill out a little more of file I/O.
Start updating ops to use new IO scheme.

Not everything is moved over or implemented yet, so this breaks
things massively.
Add missing interface implementation.
Get stdout/stderr using new IO approach.
Fix a thinko in slurp.

With this, all NQP tests pass, though there's still some notable bits
still to do.
Implement FileHandle.readline.
Get stdin and readline interactive using new IO.
Cleanup old IO leftovers.
Fix FileHandle print.
Merge branch 'io-refactor'
Fix thinko in stdout/stderr print.