IGet instruction

[lausek]

Implement new instruction IGet(u16) for retrieving items by index .get_by_index(usize).

• This would save space when working with frequent indices like list[0] and list[1]
• Also helpful when unboxing iterations over Dict:

              | stack
CPush         | [dict]
IterCreate    | [iter]
...
IterNext      | [list]
Duplicate     | [list, list]
IGet(0)       | [list, key]
LMove         | [list]
IGet(1)       | [value]
LMove         | []