Refactor VRF not to expose output of VRF function in two representations

[Harrm]

To be precise, VRFOutput and raw output byte array. Also, verify function now also checks if the value less than the threshold.

[burdges]

VRFOutput is kinda poorly named. It's really a preoutput, the actual output can only be computed with the VRFInOut::make_* methods.