Efl.Io.{Queue,Buffer,Buffered_Stream}: slice_get is now a property.

Efl.Io.{Queue,Buffer,Buffered_Stream}: slice_get is now a property.

Previously we couldn't return a slice, instead required the user to
pass a slice and we'd fill it since Eolian couldn't generate fallbacks
for structures.

Since @q66 fixed eolian, we can now return the structure itself as
initially wanted, ditching some TODO from the code.