Added stub of PreMirror.

I'm still not sure exactly what the Mirrors should look like, but
I need Mirrors (or something like them) to get Byeloblog working;
dealing with code manipulation is somewhat difficult without them.
As an added complication, the execution model is still somewhat
fuzzy, so I have inadvertantly ended up with 'explosive' values,
sort of the evil opposite of the idempotent parts of code,
implied by overzealous handling of 'decaying' parts of code -
that is, things like interpolated strings or quasiquotes.