* Move Proof and Difficulty types to pow module. Difficulty scaling calculation.
* Diffculty scaling and size shift for 2nd PoW
* Backport e7eb26ee
* Test compilation fixes
* Scaling only need to go one way, as @tromp pointed out
* Check txhashset content before zip/unzip
* Add header in txhashset verification
* Add copy function and test
* Add file util
* Now check and remove unexpected files instead of just crashing
Antioch Peverell