Less error prone EdDSA verification building blocks

Less error prone EdDSA verification building blocks

crypto_eddsa_r_check() is replaced by crypto_eddsa_check_equation().
This has two advantages:

- Users now only need to return the value of crypto_eddsa_r_check().
  No need for an additional check we may forget, much safer.
- Verifying the equation give better optimisation opportunities.