Script destinations break-out

More work on reducing script dependencies. I'm not sure if this is wanted yet, if not I'm fine with holding off.

This allows CScript to work without knowledge of CTxDestination. Most of the dependencies on key.h are dropped, and a boost dependency is dropped as well. Next obvious steps would be to do the same for CPubKey, move SetMultisig() out, etc. This seemed like a good stopping point before continuing with those.