Many useful properties are probably still missing.

This allows us to factor out properties about connectives that
commute with the big operators.

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.