Remove `set_all`, `set_axis`, and `get_axis` methods from Vector2/2i/3/3i/4/4i

[aaronfranke]

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set_all is not a very useful method, it was only used in one place in BVH. However, the rest of the BVH code just iterates over the axis count, so this PR changes the set_all code to use this approach and removes set_all. Also, set_all was broken in Vector4, which is what got me investigating this method in the first place.

I removed the set_axis and get_axis methods because they are not used internally anywhere (except for the tests) and they do the same thing as the array operator anyway.