Add initial support for namespace-aliases and namespace-imports. They are both stored into the code-repository and code-model now, and used through the namespace-aliasing system. Create facilities for efficient management and comparison of include-files for future header-parsing. Make the namespace-system aware of those facilities.

Together with some simple header-parsing (included headers need to be collected), and with a little work on making the resolution-system aware of those header-sets, that will allow near-perfect code-completion. A few other fixes.