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.

Interpreter will abort script execution on first error found.

Single variable cannot have many values of different types. This also saves some memory, because union allocates memory just for one of its members.

Every chunk should has its own file associated, even if it is [MEMORY]. Otherwise, some false information can be provided by interpreter to developer.

This is a new memory subsystem implementing heap calculations as well as new builtin functions:
 * get_memory_usage()
 * get_memory_peak_usage()
 * get_memory_limit()
It also allows to set an upper memory limit, ensuring that processed script will not be able to allocate more memory from OS.
New subsystem is based on work done in 'memory_limit' branch. Big thanks to devnexen!
This finally fixes #25.

* some functions were not used and have been removed
 * some parameters seemed to be useless and have been removed
 * code cleanup