Add parse_and_validate() for validating a query without executing it

[ChrisJr404]

Resolves #726.

Right now there's no way to tell whether a request is a query, mutation or subscription without trying to execute it, which is why juniper_graphql_ws currently has to run both execute and resolve_into_stream and see what works. This adds a parse_and_validate() that runs the same parsing and validation as execute/execute_sync but stops before resolving anything, and returns the OperationType of the operation that would run, so a caller can route to the right executor themselves.

It's purely additive, the existing execute functions are untouched. Added tests covering the three operation types plus the parse, validation and multiple-operations error paths, and a CHANGELOG entry.