Calling v.ScalarMult on a receiver v that is not the identity point results in an incorrect operation.
This was fixed by setting v to the identity point in ScalarMult.
A simple test was added to check this behaviour.
I could not decide if they should be called SetIdentity/SetGenerator, so
instead I removed them. Turns out we only needed them in one place,
where Set(NewIdentityPoint()) inlines well enough that it should perform
the same.
Most the Identity calls were redundant as the value was overwritten
before being used next.
Also, replaced Bytes (which appended, unlike big.Int.Bytes) with
FillBytes. ristretto255 has Encode/Decode instead of
FillBytes/FromCanonicalBytes in order to match Element, which is not
relevant here.
