Tighten up INCLUDE rules

The `INCLUDE` statement was originally conceived as a C-style "paste of text", although there are already restrictions on this (it separates "top level content" from knot definitions, for example). Therefore it can probably still be abused in weird ways / places, and this should be tightened up.

I'd suggest that we should turn the behaviour into an "import" style statement, where it automatically only gets included once as well.