Support subscriptions

This is experimental support.
The implementation is based on conduit and is boring. There is a new
resolver data constructor that should create a source event stream. The
executor receives the events, pipes them through the normal execution
and puts them into the response stream which is returned to the user.

- Tests are missing.
- The executor should check field value resolver on subscription types.
- The graphql function should probably return (Either
  ResponseEventStream Response), but I'm not sure about this. It will
  make the usage more complicated if no subscriptions are involved, but
  with the current API implementing subscriptions is more
  difficult than it should be.

CHANGELOG.md

@@ -22,16 +22,21 @@ and this project adheres to
 - `Error.Response` represents a result of running a GraphQL query.
 - `Type.Schema` exports `Type` which lists all types possible in the schema.
 - Parsing subscriptions (the execution always fails yet).
+- `Error.ResponseEventStream`, `Type.Out.Resolve`, `Type.Out.Subscribe` and
+  `Type.Out.SourceEventStream` define subscription resolvers.
 
 ## Changed
-- `Trans.ActionT` has become to `Type.Out.ResolverT`. Since `Type.Out.Resolver`
-  has gone it is a better name for GraphQL resolvers.
+- `Type.Out.Resolver`: Interface fields don't have resolvers, object fields
+  have value resolvers, root subscription type resolvers need an additional
+  resolver that creates an event stream. `Resolver` represents these differences
+  now and pairs a field with the function(s).
 - All code from `Trans` is moved to `Type.Out` and exported by `Type` and
   `Type.Out`.
 - `AST.Core` contained only `Arguments` which was moved to `Type.Definition`.
   `AST` provides now only functionality related to parsing and encoding, as it
   should be.
-- `Execute.execute` takes an additional argument, a possible operation name.
+- `Execute.execute` takes an additional argument, a possible operation name
+  and returns either a stream or the response.
 - `Error` module was changed to work with dedicated types for errors and the
   response instead of JSON.
 - `graphqlSubs` takes an additional argument, the operation name. The type of
@@ -40,7 +45,9 @@ and this project adheres to
   underlying functions (in the `Execute` module).
 
 ## Removed
-- `Type.Out.Resolver`: It is an unneeded layer of complexity. Resolvers are a
+- `Trans.ActionT` is an unneeded layer of complexity. `Type.Out.Resolver`
+  represents possible resolver configurations.
+- `Type.Out.Resolver`: It . Resolvers are a
   part of the fields and are called `Trans.ResolverT`.
 - `Execute.executeWithName`. `Execute.execute` takes the operation name and
    completely replaces `executeWithName`.