Added safe_open that can create file descriptors

As a follow-up to the recently merged sandbox changes, I realized we provided no safe way of reading files directly (necessary for parsing json etc. where the functions expect a file descriptor or a string). This implements open using the io_guard wrappers to ensure that it isn't called in an unsafe manner. I've also added a few other missing builtins such as list, set and frozenset.

TODO: Tests!

• Should test both reading and writing in the different contexts (global scope, src_unpack/can_update_live, src_prepare/src_install).