Add JSON support to UUID

[lachlan]

To support UUIDs being used directly in JSON mappings, I'd like to add the following methods to the standard library:

• UUID.new(JSON::PullParser)
• UUID#to_json(JSON::Builder)

I was thinking we'd add these to a new source file uuid/json.cr, and so you'd have to require "uuid/json" to opt in to the JSON support, similar to big/json.cr.

If this approach and feature is acceptable, I'd like to create a pull request for it.

[Sija]

Since #5551 has been merged, this can be closed now.