Add function to retrieve the maximum stack size.
Add class to determine the current stack size.
Use class hierarchy for the different types of expressions.
Merge branch 'stack-size-hack' of github.com:openscad/openscad into stack-size-hack

Conflicts:
	src/expr.cc
Merge branch 'stack-size-hack' of github.com:openscad/openscad into stack-size-hack
Remove duplicate warning message.