function _setbit(n, c, v) {
        var i = c >> 4;
        var a = n[i];
        a = a + (1 << (c & 0xf)) * v;
        n[i] = a;
    }
@exports jodid25519/core
Core operations on curve 25519 required for the higher level modules.

@description
Core operations on curve 25519 required for the higher level modules.

<p>
This core code is extracted from Michele Bini's curve255.js implementation,
which is used as a base for Curve25519 ECDH and Ed25519 EdDSA operations.
</p>