LibCrypto: Replace {Unsigned,Signed}BigInteger impl with LibTomMath

Replace the implementation of maths in `UnsignedBigInteger`
and `SignedBigInteger` with LibTomMath. This gives benefits in terms of
less code to maintain, correctness and speed.

These changes also remove now-unsued methods and improve the error
propagation for functions allocating lots of memory. Additionally, the
new implementation is always trimmed and won't have dangling zeros when
exporting it.

Libraries/LibCrypto/BigInt/TommathForward.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2025, Altomani Gianluca <altomanigianluca@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+extern "C" {
+typedef uint64_t mp_digit;
+typedef int mp_sign;
+
+// This is a workaround for the fact that Tommath doesn't have a proper
+// header file. It uses an anonymous struct to define the mp_int struct
+// which makes it impossible to forward declare.
+// Declare the mp_int struct with the same layout as the one in tommath.h
+// and check that the layout is the same while avoiding a conflicting
+// definition error (BN_H_ is defined in tommath.h).
+// When importing this file it is important to always do it AFTER tommath.h.
+
+typedef struct {
+    int used, alloc;
+    mp_sign sign;
+    mp_digit* dp;
+} mp_int_;
+
+#ifndef BN_H_
+typedef mp_int_ mp_int;
+#else
+static_assert(sizeof(mp_int_) == sizeof(mp_int));
+static_assert(offsetof(mp_int_, used) == offsetof(mp_int, used));
+static_assert(offsetof(mp_int_, alloc) == offsetof(mp_int, alloc));
+static_assert(offsetof(mp_int_, sign) == offsetof(mp_int, sign));
+static_assert(offsetof(mp_int_, dp) == offsetof(mp_int, dp));
+#endif
+}