Duplicate code in the curve25519_base function

[finfet]

The curve25519_base and curve25519 functions differ by only 2 lines of code. While the curve25519_base function does contain a small optimization, it is likely not worth the maintenance burden to have two different versions of this function, especially when considering that this function contains complicated, security sensitive multiplications.

I would suggest just calling curve25519 from within curve25519_base. This call will likely get optimized out by the compiler so that there is no additional performance penalty. I am happy to submit a pull request for this and discuss further.