Add ip4_to_long builtin

Add a built-in function that converts a string in IPv4 dotted notation,
with an optional bitmask (1.2.3.4/24) into a list of longs.
Document the ip4_to_long built-in
Add a long_to_ip4 built-in.

It takes a long and returns a string representing that IP in dotted notation.
Document the long_to_ip4 built-in.