This commit also adds new controls to variables, which prevents from re-assigning a value when set. Thanks to that, both $this and $parent cannot be overwritten by using OP_STORE instruction. Other instructions still need some
work.

In AerScript, there are no such language constructs, that have syntax similiar to functions. If so, it is just a builtin function.

There are several changes in this commit:
 * first of all constants declared by 'const' statement should be local (declared in current scope / frame),
 * constants are declared by using OP_DECLARE instruction,
 * OP_LOADC browses both global and local constants container,
 * PH7_VmRegisterConstant() allows both global and local declarations.
Since this commit, there are 3 kinds of constants:
 1) global
 2) local (in loop, closure, method)
 3) class members.
Actually there is no way to declare a global constant except the built-in constants.

In AerScript, the foreach() loop is syntatically more similiar to C#, than PHP. However the optional '$key => $value' construct is still available, because arrays in AerScript are still a hashmaps.