consensus lib work: split CPubKey and friends out of key.o

This is the next-to-last major movement necessary for libbitcoinconsensus. After this and version movement (see next PR), only script interpreter error handling remains.

Split CPubKey and friends out so that they can be used without CKey's dependencies. Stub files have been created for handling crypto functionality without any core deps (eccryptoverify.h/cpp). Eventually these will handle crypto-level key functions, and CKey/CPubKey will call into them. Because some code changes are needed for that, and it's not strictly necessary for the verification lib for 0.10, that change has been skipped here.
ACK on adding the null pointer check here (although it's not move-only)