Add schema introspection support and execution-related APIs

[SimonSapin]

Changelog:

• Add data structure in apollo_compiler::execution for a GraphQL response, its data, and errors.
  All (de)serializable with serde.
• Add coerce_variable_values() in that same module.
• Add apollo_compiler::execution::SchemaIntrospection
  providing full execution for the schema introspection parts of an operation
  and separating the rest to be executed separately.
  In order to support all kinds of introspection queries this actually includes
  a full execution engine where users provide objects with resolvable fields.
  At this time this engine is not exposed in the public API.
  If you’re interested in it let us know about your use case!
• Add ExecutableDocument::insert_operation convenience method.

---

Replaces and closes #645:

• The execution engine and introspection resolvers are copied from there. async was removed (since unused by introspection, the only user)
• The public API was reworked to be part of apollo-compiler
• The logic splitting an operation into introspection and non-introspection parts was rewritten two produce valid documents. This logic needs a lot more testing.

[SimonSapin]

Converted to draft for:

• Error representation. Currently RequestError contains a GraphQLError with line and column numbers. Maybe it should instead be something that can convert to GraphQLError.
• Handling of schema introspection fields not at the top level is probably broken. I think we should make them invalid.

https://spec.graphql.org/October2021/#sec-Schema-Introspection

The schema introspection system is accessible from the meta-fields __schema and __type which are accessible from the type of the root of a query operation.

This could say from the root of a query, but it says from the type of the root of a query, which suggests this is valid:

schema { query: Fibonacci }
type Fibonacci { value: Int, next: Fibonacci }

query { value next { value next { __schema { description }}}}

I suspect this is not intended.

[SimonSapin]

I pushed a rework of error handling. introspection_split.rs still needs work to reject non-top-level introspection, but the rest of the PR can be considered ready for review.

[lrlna]

We should probably merge this in without making too many more changes, and then incrementally improve after we get a chance to try it out in the router (like you said before, Simon).

Ideally we'd try integrating it in the router, see if our users have any feedback, implement tests from graphql-js implementations and make this API a 1.0 with the compiler. But yea, regardless, let's get this in.

[lrlna]

sometime later we should port over the graphql-js introspection & execution tests

[SimonSapin]

Alright, this is now fully ready for review! Please also take a look at introspection_split.rs and its tests.

[lrlna]

I think you should merge, @SimonSapin ^__^