Internally to mutable, there is a data structure that holds a fixed size array of 'fast' ElementRep objects. If that array is filled, we then spill to a std::vector.

One side effect of this is that tests that only manipulate a small number of elements never hit the std::vector.

We want our testing to test both modes. The easiest way to achieve this is to select a very small size for the 'fast' objects in debug builds.