compiler: implement preprocessor expressions (fixes mantis #0010671)
  - move operator_levels to topens.pas - it is used from 2 units now
  - implement pexpr like sub_expr for preprocessor expressions
  - implement +,-,*,/ expressions for the moment
  * move OR, AND, IN implemenetation to the new logic

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@25465 3ad0048d-3df7-0310-abae-a5850022a9f2