Mechanisms for executing parsed queries against compiled schemas.

Adds improved spec messages to Lacinia specs.

Parse a query document using a compiled schema.

Also provides functions that operate on the parsed query.

Parsing of a Markdown file for purposes of attaching documentation to a schema.

Parse a Schema Definition Language document into a Lacinia input schema.

Complex results for field resolver functions.

Resolver functions may return a value directly, or return a value in an immediate
or asynchronous [[ResolverResult]].  The resolved value may be wrapped in a
a modifier:

* [[with-error]]
* [[with-warning]]
* [[with-context]]
* [[with-extensions]]

The modifiers exist to resolve a value _and_ to perform a side effect, such as
adding an error to the execution result.

A value or wrapped value may be returned asynchronously using a [[ResolverResultPromise]].

The [[FieldResolver]] protocol allows a Clojure record to act as a field resolver function.

Responsible for constructing and validating the GraphQL schema.

GraphQL schema starts in a format easy to read and maintain as an EDN file.

Compiling the schema performs a number of validations and reorganizations to
make query execution faster and simpler, such as generating a flatter structure for the
schema, and pre-computing many defaults.

Protocols for selection-related objects accessible from the [[selection]] function, and
as well as the schema type objects navigable to from the selection.

Support for tracing of requests, as per the
[Apollo tracing specification](https://github.com/apollographql/apollo-tracing).

Useful utility functions.