Port Bitcoin's key.cpp and pubkey.cpp code

[barton2526]

While investigating bitcoin/bitcoin#16878 I found that our CKey code differs significantly from upstream's, particularly in the Set and Check methods (we do not have these at all). This leads to upsteam's file pubkey.cpp, which we do not have either. It was created here: bitcoin/bitcoin#5162.

Many of these differences date to 2013 or 2014, which should be before our split from upstream, but could be vestigal Black/Peercoin code. For future porting and security, we should strive to bring our key code in line with Bitcoin.