Converts full proof bytes to quality string (does not validate the proof).
Returns Some(quality) on success, None if proof format is invalid or conversion fails.
out must point to exactly 129 bytes
serializes the QualityProof into the form that will be hashed together with
the challenge to determine the quality of ths proof. The quality is used to
check if it passes the current difficulty. The format is:
1 byte: plot strength
repeat 16 times:
8 bytes: little-endian proof fragment