LibCrypto: Add a simple SignedBigInteger

This patchset adds a simple SignedBigInteger that is entirely defined in
terms of UnsignedBigInteger.

It also adds a NumberTheory::Power function, which is terribly
inefficient, but since the use of exponentiation is very much
discouraged for large inputs, no particular attempts were made
to make it more performant.

Libraries/LibCrypto/CMakeLists.txt

@@ -1,5 +1,6 @@
 set(SOURCES
     BigInt/UnsignedBigInteger.cpp
+    BigInt/SignedBigInteger.cpp
     Cipher/AES.cpp
     Hash/MD5.cpp
     Hash/SHA1.cpp