Factor scalar test vectors into lib/rbnacl/test_vectors.rb

Experimenting with having a single place for all test vectors
Store all test vectors as hex

After playing around with some conversions on the test vectors, I feel
it's probably best they're both stored and represented as hex.

This also shows a use case for supporting the encoding API inside
Crypto::Points. Without it I can't use the nice hex test vectors ;)

I'd call this cleaner and less convoluted in all regards.
Use Util.check_length for Points