Apply patch from upstream SVN r4641

BUG fix: the optimizer created incorrect code for subtraction a variable from a
constant.

https://code.google.com/p/pawnscript/source/detail?r=38

Fixes #10.
Better stringlist fix

The previous didn't quite work with the docstring list. I also renamed
the insert_* functions to push_* to reflect the fact that they append
the list and delete_* to popfront_* as delete_docstring was used for
removing only the first element.
Fix lots of compile warnings