Made wire_func_t not have virtual functions.
Maybe maybe_wire_func_t use composition instead of private inheritance.
Renamed to maybe_wire_func_t::compile_wire_func_or_null.
Removed unused function map_wire_func_t::make_safely.
Updated out-of-date comment.
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next