Move most consensus constants and consensus function declarations to consensus/consensus.h That includes:
-Most consensus constants that are not in script already -Functions declaration for most of what's necessary for verifying transactions, block headers and blocks
This commit will allow us to separate libconsensus code more easily and also, in combination with #5669, start a big include cleanup (specially if we do some similar moves for policy in between).
This is the first step in #6051, see that PR for a more detailed plan on short term moveonly-related changes.