Rename native to src

With the removal of the host, this submodule is now just one thing:
libpsnative, so it makes sense to have a `src` directory instead of a
poorly named `monad-native/native`.

CMakeLists.txt

@@ -5,5 +5,5 @@ add_compile_options(-std=c++11 -Wall -Werror)
 
 # test in BUILD_DIR
 enable_testing()
+add_subdirectory(src)
 add_subdirectory(tests)
-add_subdirectory(native)