code/Include: Superfluous line in code section
Add code/Variables{SingleFile,MultipleFiles}, variables tests
code/Variables{SingleFile,MultipleFiles}: more test cases
Fix build
code/Rule: Another test case
Rework variable management, so the tests are passed

* Assigment: "?=" applies also when the variable is already defined, but
  empty.
* Block: Push a local variable scope, unless the parent node already did
  that (in the case of UserRuleInstructions).
* EvaluationContext: There are no longer two stacks of variable scopes
  (global and local). The global scope is just a flat scope. There is a
  new variable domain for built-in variables (the number variables and
  "<"/">" for rule parameters), which are set by UserRuleInstructions.
LocalVariableDeclaration: Fix crash, the initializer can be NULL
EvaluationContext: Get rid of the global scope entirely

Access to the global variable domain suffices.