LibCrypto: Add the montgomery modular power algorithm

This algorithm allows for much faster computations of modular powers
(around a 5x-10x speedup of the Crypto test). However, it is only valid
for odd modulo values, and therefore the old algorithm must be kept for
computations involving even modulo values.

Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.h

@@ -58,6 +58,7 @@ public:
         m_cached_trimmed_length = {};
     }
 
+    bool is_odd() const { return m_words.size() && (m_words[0] & 1); }
     bool is_invalid() const { return m_is_invalid; }
 
     size_t length() const { return m_words.size(); }