Implement json.Marshaller interface for Inet, Macaddr (et al.)

[atz]

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Application JSON data coming in (or going out) should not know (or reveal) anything about our underlying implementation model. For a given struct like:

type My struct {
  IP pgtype.Inet
  MAC pgtype.Macaddr
}

Populated accordingly, it would be reasonable for json.Marshall to deliver bytes equivalent to (for example):

{"IP":"192.0.2.1/32","MAC":"00:0a:95:9d:68:16"}

Symmetrically, json.Unmarshal should be able reconstitute the struct from the serialized JSON. Can probably submit a PR for some of this if you concur. Currently that would give an error like:

json: cannot unmarshal string into Go struct field My.IP of type pgtype.Inet

[jackc]

I think that would work.

Only thing to watch out for is ambiguous values that can't be round-tripped or things that don't have an obvious JSON representation. That has been an obstacle for JSON marshalling support for some other types. But off the top of my head I don't see any issues here.