Add safegcd based modular inverse modules

Refactored by: Pieter Wuille <pieter@wuille.net>

src/modinv32.h

@@ -0,0 +1,31 @@
+/***********************************************************************
+ * Copyright (c) 2020 Peter Dettman                                    *
+ * Distributed under the MIT software license, see the accompanying    *
+ * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef SECP256K1_MODINV32_H
+#define SECP256K1_MODINV32_H
+
+#if defined HAVE_CONFIG_H
+#include "libsecp256k1-config.h"
+#endif
+
+#include "util.h"
+
+typedef struct {
+    int32_t v[9];
+} secp256k1_modinv32_signed30;
+
+typedef struct {
+    /* The modulus in signed30 notation. */
+    secp256k1_modinv32_signed30 modulus;
+
+    /* modulus^{-1} mod 2^30 */
+    uint32_t modulus_inv30;
+} secp256k1_modinv32_modinfo;
+
+static void secp256k1_modinv32(secp256k1_modinv32_signed30 *x, const secp256k1_modinv32_modinfo *modinfo);
+static void secp256k1_modinv32_var(secp256k1_modinv32_signed30 *x, const secp256k1_modinv32_modinfo *modinfo);
+
+#endif /* SECP256K1_MODINV32_H */