Verify against difficulty

[chfast]

The final hash verification is

final_hash <= 2^256 / difficulty

The current verify() API takes boundary = 2^256 / difficulty instead of difficulty. The boundary is commonly used in mining for efficiency. However, for block hash verification this is not optimal because clients have to compute 2^256 / difficulty inversion for every block hash.

We propose to add additional function which takes difficulty argument directly: verify_against_difficulty().
Internally, we can perform much more efficient check:

final_hash * difficulty <= 2^256

This has been tested and the formula is also valid for difficulty 0 and 1.

We also propose to rename verify() to verify_against_boundary() to disambiguate.
And change verify_final_hash() to verify_final_hash_against_difficulty() as this function is suppose to be used for quick block hash verification.

[AndreaLanfranchi]

I'm totally in favor of this.