Negative indexing is a useful feature in Python, especially when combined
with array slicing. Array slicing will hopefully be implemented later, but
negative indexing is useful in its own right.
A negative index is indexing from the end of an array,
"array[-1] == array[array.size()-1]", using a negative index
larger/smaller than the length of the array is still an error.
While primarily useful for arrays and strings, support is also added to
"array like" structures like Vector3 and Color. This is done just
to be consistent; vector3[2] is much clearer than vector3[-1], but disallowing
it while allowing it for an array with 3 elements seems confusing.
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