This is (almost) drop-in replacement for std::vector, the main
difference is that FixedCapacityVector has a fixed (at runtime)
capacity, which makes it a lot more efficient and small for operations
like push_back();
The capacity can be set (and re-set) as usual with reserve(),
but exceeding the capacity when adding items will throw.
The capacity is also set when creating the vector with a size.
In addition a FixedCapacityVector<> instance is only 16 bytes instead
of 24 for std::vector<>.
The aim here is to reduce code bloat.
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