Convert `interval` to a pojo and use object pooling.

[lfn3]

The structure of intervals is pretty fixed, and getting increasingly so.
It would be possible to change them into pojo's, and then use object pooling to prevent allocations.
Since we create and destroy a large number of intervals during the lifecycle of a single test run
(at least one per generated value at the moment) it might have a significant performance impact.
Between this and eliminating as many intervals as possible, I should be able to reduce the overhead of using undertaker significantly.