Avoids dragging in a bunch of includes from the header files, and also
reduces the amount of files that need to be recompiled if one of those
included headers' source content is ever changed.
Previously there was only one function under the NetPlay namespace,
which is kind of silly considering we have all of these other types
and functions existing outside of the namespace.
This moves the rest of them into the namespace.
This gets some general names, like Player, for example, out of the global namespace.
