MOAR Lua frontend performance!

- Change native interpolation code to ignore $(<) and $(@) unless there is a
  lookaside table passed in.

- Interpolate again from cached key to resolve < and @ only.

- Invalidate cache when environment is changed.

Gives a good 1 second speedup on one of my very big testcases.

Bonus:
Don't crash if unable to write profile file.