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/Tommath.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2025, Altomani Gianluca <altomanigianluca@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+
+#include <tommath.h>
+
+inline ErrorOr<void> mp_error(mp_err error)
+{
+    switch (error) {
+    case MP_OKAY:
+        return {};
+    case MP_MEM:
+        return Error::from_errno(ENOMEM);
+    case MP_VAL:
+        return Error::from_errno(EINVAL);
+    case MP_ITER:
+        return Error::from_string_literal("Maximum iterations reached");
+    case MP_BUF:
+        return Error::from_string_literal("Buffer overflow");
+    default:
+        return Error::from_string_literal("Unknown error");
+    }
+}
+
+#define MP_TRY(...) TRY(mp_error((__VA_ARGS__)))
+
+#define MP_MUST(...) MUST(mp_error((__VA_ARGS__)))