Move StringSpaceUsedIncludingSelf to lite library.

This came up because Chromium downstream modifies the lite library in a way that
requires this function, but I'm upstreaming it because based on the comments in
repeated_field.h, this ought to allow resolution of an existing hack.

I don't know enough about the protobuf code to feel confident trying to resolve
this hack myself, so I've merely updated the TODO comments.