Added const type support to compose compiler, api meta data now auto-generated
- Variables, fields and parameters can now be declared const in Compose
  scripts
- added codegen_api.py which autogenerates system_api_meta.cpp based
  upon contents of system_api.h.
- experimented with making all system_apis return an error code, and
  take the "return value" as the first parameter
- Turned out to be a bad idea since it changes the semantics of a bunch
  of expression types (like *=, +=, etc), and makes it much more
  difficult to generate C++ from Compose scripts
- Instead I plan to pursue raising errors in entities from the apis and
  have the Entity class act appropriately when an error is raised