More rez.rex refactors. Mostly naming and documentation.
- Commands are not Actions, to avoid confusion with the command() command.
- CommandExecutor is now the ActionManager, since it now acts as a middleman to the ActionInterpreter
- RexNamespace is now RexExecutor, since it is no longer a namespace dict and now manages the actual execution of via python exec
rez-interpret: a prototype for speeding up runtime interpretation of pickled resolves.
- takes the path to a pickled ResolveResult
- uses new rez-daemon command to spawn a daemonized server process which runs the rex interpretation and returns it via netcat.  by avoding python startup time it provides over 10x speed boost.