This patch adds the function JSON_SCHEMA_VALID(<json doc>, <json doc>) that
validates a JSON document against a JSON schema. A JSON schema is a
JSON document that describes the general structure of a JSON document.
The JSON schema can be used to enfore different things like:

- Required properties
- Numbers within a range
- Strings must match a regular expression
- And much more

The JSON Schema validator uses std::regex in order to do pattern
matching in those cases where it's used. It rejects JSON Schema
definitions that contain references to remote documents. In order to
re-use some of the JSON syntax checking, we move some existing code
out of json_dom.h/cc and into a separate file json_syntax_check.h/cc.

Change-Id: Idde632ae11c5ba8992e77e77f214e74ecfe367c3