Add scalar blinding and a secp256k1_context_randomize() call.

This computes (n-b)G + bG with random value b, in place of nG in
 ecmult_gen() for signing.

This is intended to reduce exposure to potential power/EMI sidechannels
 during signing and pubkey generation by blinding the secret value with
 another value which is hopefully unknown to the attacker.

It may not be very helpful if the attacker is able to observe the setup
 or if even the scalar addition has an unacceptable leak, but it has low
 overhead in any case and the security should be purely additive on top
 of the existing defenses against sidechannels.

src/ecmult_gen.h

@@ -24,6 +24,8 @@ typedef struct {
      * the intermediate sums while computing a*G.
      */
     secp256k1_ge_storage_t (*prec)[64][16]; /* prec[j][i] = 16^j * i * G + U_i */
+    secp256k1_scalar_t blind;
+    secp256k1_gej_t initial;
 } secp256k1_ecmult_gen_context_t;
 
 static void secp256k1_ecmult_gen_context_init(secp256k1_ecmult_gen_context_t* ctx);
@@ -36,4 +38,6 @@ static int secp256k1_ecmult_gen_context_is_built(const secp256k1_ecmult_gen_cont
 /** Multiply with the generator: R = a*G */
 static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context_t* ctx, secp256k1_gej_t *r, const secp256k1_scalar_t *a);
 
+static void secp256k1_ecmult_gen_blind(secp256k1_ecmult_gen_context_t *ctx, const unsigned char *seed32);
+
 #endif