Add FromJSON / ToJSON instances for Schema

[reactormonk]

So a schema can be parsed / dumped.

[axman6]

I'd love to see this happen, I was all set to use this package until I realised I couldn't use it to read a schema and validate it.

[bergmark]

In case you aren't aware: This package isn't related to the JSON schema spec, this is something custom we built and unfortunately it ended up name clashing with something else.

[axman6]

That's sad news, I should've looked into it further! :)

[bergmark]

Last time i looked for implementations of the spec i found aeson-schema and hjsonschema but they seem to be outdated or unmaintained, unfortunately.