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This way we can more easily state lemmas for concrete languages for
arbitrary global functors.

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.

This reverts commit 5161e463.

This way, they are non-delta unfoldable constants, which showed a positive
impact on the performance of setoid rewriting. We may want to do this for
other cmra/cofe structures too.