1. rewrite of the Haven variation of the Pricing Record class to support:
- versioning of the PR format
- nested supply_data and asset_data structs
- verification of the signature using a variable-length string (not R+S)
2. calculation of the slippage tallies for a block in add_block(), so that
we can work out the yield that is due to be paid out for the block.
Loads of little fixes and cleanups.
Lee Clagett