This PR implements range iterators in the base containers (Vector, Map, List, Pair Set).
Given several of these data structures will be replaced by more efficient versions, having a common iterator API will make this simpler.
Iterating can be done as follows (examples):
```C++
//Vector<String>
for(const String& I: vector) {
}
//List<String>
for(const String& I: list) {
}
//Map<String,int>
for(const KeyValue<String,int>&I : map) {
print_line("key: "+I.key+" value: "+itos(I.value));
}
//if intending to write the elements, reference can be used
//Map<String,int>
for(KeyValue<String,int>& I: map) {
I.value = 25;
//this will fail because key is always const
//I.key = "hello"
}
```
The containers are (for now) not STL compatible, since this would mean changing how they work internally (STL uses a special head/tail allocation for end(), while Godot Map/Set/List do not).
The idea is to change the Godot versions to be more compatible with STL, but this will happen after conversion to new iterators have taken place.
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