Generalise collision damage; use in more places.

Knocking something back into a solid feature or another creature now
hurts it; this includes Force Lance, the fan of gales, and other things
that knock you back.

This also handles cases of multi-square knockbacks more gracefully.

Possible extra case that isn't handled right now: trampling.
Tinker with Force Lance.

It is now L4 Conj/Tloc and appears additionally in the Book of the Warp;
it is slightly more accurate (as accurate as IMB, which I think is
acceptable based on its short range), and can knock back enemies up to
six tiles at full power (scaling with power).