verify_proof/4 checks that a solution is a valid solution it doesn't check
it against any target.
get_target/2 returns the target of a solution in integer form
(not scientific).
aecuckoo_SUITE is moved from aecuckoo to aeminer
To make the suite pass:
- siphash24 module was taken from ae node
- enacl has become a dep of aeminer (due to blake2b_256 hash)
