LibCore: Add SecretString, a buffer that is zero'd on destruction

We have a few places where we read secrets into memory, and then
do some computation on them. In these cases we should always make
sure we zero the allocations before they are free'd.

The SecureString wrapper provides this abstraction by wrapping a
ByteBuffer and calling explicit_bzero on destruction of the object.

Userland/Libraries/LibCore/CMakeLists.txt

@@ -24,6 +24,7 @@ set(SOURCES
     Process.cpp
     ProcessStatisticsReader.cpp
     Property.cpp
+    SecretString.cpp
     Socket.cpp
     StandardPaths.cpp
     TCPServer.cpp