+ Backport of python 3 enum functionality.
+ Update to use new enum based representation of state.
+ Improve doc strings.
+ Update to use new enum based representation of state.
+ Update tests based on enum changes.
+ Fix up missed reference to old-style status
Merge remote-tracking branch 'mark/resolved_context_status_to_enum' into testing_1

Conflicts:
	src/rez/build_process.py
	src/rez/resolved_context.py
	src/rez/resolver.py
	src/rez/solver.py