encoding/jsonschema: implement more format constraints

[rogpeppe]

The format keyword in jsonschema could be implemented more fully.

As a very rough guide to prioritization, here are the set of formats used in schemastore.org,
each with the count of how many schemas use that keyword (ignoring different versions
of the same schema):

82   uri
26   date-time
20   uri-reference
14   regex
12   email
7    date
3    uuid
2    hostname
1    time
1    iri
1    ipv6
1    ipv4
1    int64
1    int32
1    duration
1    double
1    byte

Note that some of the formats used above are not actually recognized in JSON Schema. For example, int32 and int64 are OpenAPI-specific.