gapi: langpuppet: Add initial implementation

This new entrypoint allows graph generation from both a Puppet manifest
and a piece of mcl code. The GAPI implementation wraps the two existing
GAPIs.

examples/langpuppet/graph1.pp

@@ -0,0 +1,10 @@
+class mgmt_first_handover {}
+class mgmt_second_handover {}
+
+include mgmt_first_handover, mgmt_second_handover
+
+Class["mgmt_first_handover"]
+->
+notify { "second message": }
+->
+Class["mgmt_second_handover"]