Modernizes interface by not forcing users to deal with raw pointers,
without adding copying overhead. Generalizes the logic of whether
we return by value or by optional/pointer.
In cases where functions take a `const std::string&` and it would
be too much work to update them, a string copy is made (which was
already happening anyway).
Move util/message to common/signmessage so it is named more clearly, and
because the util library is not supposed to depend on other libraries besides
the crypto library. The signmessage functions use CKey, CPubKey, PKHash, and
DecodeDestination functions in the consensus and common libraries.
