[SECP256K1] Add safegcd based modular inverse modules

Summary:
Comes with full documentation and tests.

Partial backport of [[bitcoin-core/secp256k1#831 | secp256k1#831]]:
bitcoin-core/secp256k1@8e415ac
bitcoin-core/secp256k1@d8a92fc
bitcoin-core/secp256k1@151aac0

Depends on D9401.

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9402

Makefile.am

@@ -38,6 +38,10 @@ noinst_HEADERS += src/field_5x52.h
 noinst_HEADERS += src/field_5x52_impl.h
 noinst_HEADERS += src/field_5x52_int128_impl.h
 noinst_HEADERS += src/field_5x52_asm_impl.h
+noinst_HEADERS += src/modinv32.h
+noinst_HEADERS += src/modinv32_impl.h
+noinst_HEADERS += src/modinv64.h
+noinst_HEADERS += src/modinv64_impl.h
 noinst_HEADERS += src/assumptions.h
 noinst_HEADERS += src/util.h
 noinst_HEADERS += src/scratch.h

src/modinv32.h

@@ -0,0 +1,42 @@
+/***********************************************************************
+ * 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"
+
+/* A signed 30-bit limb representation of integers.
+ *
+ * Its value is sum(v[i] * 2^(30*i), i=0..8). */
+typedef struct {
+    int32_t v[9];
+} secp256k1_modinv32_signed30;
+
+typedef struct {
+    /* The modulus in signed30 notation, must be odd and in [3, 2^256]. */
+    secp256k1_modinv32_signed30 modulus;
+
+    /* modulus^{-1} mod 2^30 */
+    uint32_t modulus_inv30;
+} secp256k1_modinv32_modinfo;
+
+/* Replace x with its modular inverse mod modinfo->modulus. x must be in range [0, modulus).
+ * If x is zero, the result will be zero as well. If not, the inverse must exist (i.e., the gcd of
+ * x and modulus must be 1). These rules are automatically satisfied if the modulus is prime.
+ *
+ * On output, all of x's limbs will be in [0, 2^30).
+ */
+static void secp256k1_modinv32_var(secp256k1_modinv32_signed30 *x, const secp256k1_modinv32_modinfo *modinfo);
+
+/* Same as secp256k1_modinv32_var, but constant time in x (not in the modulus). */
+static void secp256k1_modinv32(secp256k1_modinv32_signed30 *x, const secp256k1_modinv32_modinfo *modinfo);
+
+#endif /* SECP256K1_MODINV32_H */