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/secp256k1.c

@@ -410,3 +410,10 @@ int secp256k1_ec_privkey_import(const secp256k1_context_t* ctx, unsigned char *s
     secp256k1_scalar_clear(&key);
     return ret;
 }
+
+int secp256k1_context_randomize(secp256k1_context_t* ctx, const unsigned char *seed32) {
+    DEBUG_CHECK(ctx != NULL);
+    DEBUG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
+    secp256k1_ecmult_gen_blind(&ctx->ecmult_gen_ctx, seed32);
+    return 1;
+}