Add support for sys:get_state/1,2 and sys:replace_state/2,3

R16B01 will add two debugging functions:

sys:get_state/1,2 will return the gen_process' state.

sys:replace_state/2,3 will alter the gen_process' state using the fun
passed as the second argument. The gen_process will gain this new state
and the function also returns this new state. Should the fun throw an
exception it will be caught and the current state will remain - and be
returned.