AK+LibX86: Generalize u128/256 to AK::UFixedBigInt

Doing these as custom classes might be faster, especially when writing
them in SSE, but this would cause a lot of Code duplication and due to
the nature of constexprs and the intelligence of the compiler they might
be using SSE/MMX either way

Tests/AK/CMakeLists.txt

@@ -58,6 +58,7 @@ set(AK_TEST_SOURCES
     TestTuple.cpp
     TestTypeTraits.cpp
     TestTypedTransfer.cpp
+    TestUFixedBigInt.cpp
     TestURL.cpp
     TestUtf8.cpp
     TestVariant.cpp