Added new CEP Symbol Sets

[MSoegtropIMC]

[andres-erbsen]

See also https://github.com/coq/coq/issues/6144

[herbelin]

Maybe lists of constants could (indeed) be assembled into "unfolding hints" and various set-theoretic operations could apply more generally to all kinds of hint databases, including providing automatic inclusion of all constants from a module?

[MSoegtropIMC]

@herbelin : indeed allowing set operations on auto hint databases would also be a very useful feature. If one can find a common syntax to handle "unfolding databases" and hint databases, I would appreciate it.

But please note that it should be possible to collect all symbols of a module without touching the module itself, say starting some auto collection.