Validate (syntax + type check) queries

[d12]

Presently, if one field tries to load a syntactically invalid or type-invalid query, it will break the entire batched query and no data will be loaded.

This could be avoided by validating queries before the merging step.

This will incur a performance hit though, and it'll require users to provide a schema definition for the APIs their dependent on. A good mid-way point might just be syntactically validating queries, and ignoring type-correctness. This way, we don't need an SDL.