Experimental branch to add parsing/validation support to envy.

This is dipping a toe into the water of adding parsing and validation support to envy. Envy so far only has done validation. However, more complex validators pretty much end up being parsers, and it doesn't make sense to throw away that work once done.

This patch streamlines parsing of ipv4/ivp6/hostnames to an address, a point which has caused a little bit of pain when interpreting config files.

Some of this may be simplified in R16, with the introduction of inet:parse_address