Added a backing for a CodePack object.  Using a CodePack, a user
should be able to construct endpoint objects, which they can then call
methods directly on.
Minor typo: endpoint tvar should match an Identifier ast node, not an Identifier terminal.
Lexing and parsing for service factory types.
Endpoints now come packaged with their own constructor objects.
Renamed codepack to service factory.
Changed signature for InternalServiceFactory: internal code passes in
an active event to keep the method emitting simpler for internal
compiler.
Emitting service factory types.
Added test cases to ensure that service factories worked on a single node.  Next, should try to serialize service factories and load them on foreign nodes.