Improve API

[pinepain]

This PR:

• Add support for zend_type (new in PHP 7.2);
• Raise required libv8 version to 5.7.514;
• Add V8\Value::IsNullOrUndefinedl() method;
• Add more V8\Value::Is*() methods;
• Add V8\MapObject;
• Remove V8\ObjectValue::{Get,Set,Has,Delete,CreateDataProperty}Indexed methods;
• Fix obj and func templates external memory adjusting;
• Add V8\SetObject.

As of this PR libv8 >= 5.7.514 required.

BC-breaking changes: V8\ObjectValue::{Get,Set,Has,Delete,CreateDataProperty}Indexed methods removed.