Get hash boundary from difficulty

[AndreaLanfranchi]

A nice to have feature would be to have the computation of hash256 boundary from the difficulty value as specified in block header

[chfast]

Doesn't this require uint256 arithmetic?

[chfast]

This requires division. We could copy code from intx to provide such feature, but this is a lot of complex code to copy and write Knuth division "from scratch".

However, I have alternative solution which may be even better: #185.

[chfast]

The #191 contains an experimental implementation (done for testing). But #195 is preferred solution.