Remove RA from the hierarchy.
Instead, we have just a construction to create a CMRA from a RA. This construction is also slightly generalized, it now works for RAs over any timeless COFE instead of just the discrete COFE. Also: * Put tactics and big_ops for CMRAs in a separate file. * Valid is now a derived notion (as the limit of validN), so it does not have to be defined by hand for each CMRA. Todo: Make the constructions DRA -> CMRA and RA -> CMRA more uniform.