Moves token transfer hooks into taxation module

[will-holley]

Summary

Because _beforeTokenTransfer and _afterTokenTransfer strictly deal with taxation (collecting tax and resetting tax collection statistics, respectively), these functions were moved into the taxation module.

Modifies

• Streamlines redundant inheritance: Lease.sol inherits from ILease and Taxation.
  • Misc: This leads me to believe the layers of abstraction are incorrect. PCO's three core components are self-assessed valuation, taxation, and perpetual market. Each of these modules should be interoperable with the others, according to a predefined interface, which resultingly enables business logic to be alternated (e.g. periodic instead of perpetual auctions). The current inheritance graph maintains Leasing, Taxation, and Valuation as a single extended hierarchy rather than peers.