Added a shortcut for faster _iter* calls supplied by modules.

[SleepProgger]

Referenced in Issue #7218

This commit adds the possibility to implement faster iterators over Objects in C++.
This will be handy for custom data structures (hint* linked list #7194) and modules providing anything iterable (AI, Simulation related stuff, ...) .

[SleepProgger]

Little Benchmark (release mode):
C++ impl iterator: Needed 67 ms for 1000000. 0.000067 ms per item.
C++ impl bound functions: Needed 855 ms for 1000000. 0.000855 ms per item.
GDscript iterator: Needed 1100 ms for 1000000. 0.001100 ms per item.
Array: Needed 61 ms for 1000000. 0.000061 ms per item.

Test module and gdscript

[bojidar-bg]

Looks good to me, but assigning reduz to review it (as it touches some really core stuff 😄 )

[reduz]

Sorry, it's way too hackish, the decision for GDScript is that simple code is prefered over performance, you can most likely find another way around the problem you are trying to solve