Refactor callbacks and add external allocated memory notification

[pinepain]

In this PR new methods added and some internal logic refactored

In this PR:

• add V8\AdjustableExternalMemoryInterface interface;
• add V8\ObjectValue::AdjustExternalAllocatedMemory() method;
• add V8\ObjectValue::GetExternalAllocatedMemory() method;
• add V8\FunctionTemplate::AdjustExternalAllocatedMemory() method;
• add V8\FunctionTemplate::GetExternalAllocatedMemory() method;
• add V8\ObjectTemplate::AdjustExternalAllocatedMemory() method;
• add V8\ObjectTemplate::GetExternalAllocatedMemory() method;
• rewrite callbacks structures to use std containers;
• use realistic external allocated memory value to notify isolate about based on callbacks structures size and optionally specified by user value.