In most (all?) modern OOP languages class members visibility is assumed to be private and programmer has to consciously set it to public or protected. PHP has the different approach what can cause a security flaws in written scripts. AerScript will not follow this way, as it seems to be conceptually broken.

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.