A few minor changes.

- Make module 'new' functions return slices of properties structs rather than
  just one.
- Fix a couple places where errors were being reported incorrectly.
- Add ModuleContext methods for dealing with other modules.
- Make property value unpacking skip unexported struct fields rather than
  panicing.

Change-Id: I4775ef77ff0514fc2ce5abccbe2ce534c05272f4