Thanks to Amin Timany for the suggestion.

The singleton maps notation is now also more consistent with the
insert <[_ := _]> _ notation for maps.

Also, make our redefinition of done more robust under different
orders of Importing modules.

Also do some minor clean up.

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.