Don't even think about recovering from out of memory

This replaces the `check_mem(...)` helper, that would call the usual `goto error;` trap to the new `assert_mem` which will exit with a failed assertion.

This will let me ignore an entire class of error for the cost of two things
  
  1. I can't catch OOM errors in client code
  2. OOM errors will crash the ruby(or other) interpreter in FFI mode

Not certain if the trade is worth it yet, but the code is here.