LibCrypto: Split BigInteger operations into an Algorithms class

Since the operations are already complicated and will become even more
so soon, let's split them into their own files. We can also integrate
the NumberTheory operations that would better fit there into this class
as well.

This commit doesn't change behaviors, but moves the allocation of some
variables into caller classes.

Userland/Libraries/LibCrypto/BigInt/Algorithms/Multiplication.cpp

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * Copyright (c) 2020-2021, Dex♪ <dexes.ttp@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "UnsignedBigIntegerAlgorithms.h"
+
+namespace Crypto {
+
+/**
+ * Complexity: O(N^2) where N is the number of words in the larger number
+ * Multiplication method:
+ * An integer is equal to the sum of the powers of two
+ * according to the indices of its 'on' bits.
+ * So to multiple x*y, we go over each '1' bit in x (say the i'th bit),
+ * and add y<<i to the result.
+ */
+FLATTEN void UnsignedBigIntegerAlgorithms::multiply_without_allocation(
+    UnsignedBigInteger const& left,
+    UnsignedBigInteger const& right,
+    UnsignedBigInteger& temp_shift_result,
+    UnsignedBigInteger& temp_shift_plus,
+    UnsignedBigInteger& temp_shift,
+    UnsignedBigInteger& temp_plus,
+    UnsignedBigInteger& output)
+{
+    output.set_to_0();
+
+    // iterate all bits
+    for (size_t word_index = 0; word_index < left.length(); ++word_index) {
+        for (size_t bit_index = 0; bit_index < UnsignedBigInteger::BITS_IN_WORD; ++bit_index) {
+            // If the bit is off - skip over it
+            if (!(left.m_words[word_index] & (1 << bit_index)))
+                continue;
+
+            size_t shift_amount = word_index * UnsignedBigInteger::BITS_IN_WORD + bit_index;
+
+            // output += (right << shift_amount);
+            shift_left_without_allocation(right, shift_amount, temp_shift_result, temp_shift_plus, temp_shift);
+            add_without_allocation(output, temp_shift, temp_plus);
+            output.set_to(temp_plus);
+        }
+    }
+}
+
+}