This improves performance of the procedure executor on secondary metrics (i.e. not for the main use case when many elements are processed together, but for the use case when a single element is processed at a time). In my benchmark I'm measuring a 50-60% improvement: * Procedure with a single function (executed many times): `5.8s -> 2.7s`. * Procedure with 1000 functions (executed many times): `2.4 -> 1.0s`. The speedup is mainly achieved in multiple ways: * Store an `Array` of variable states, instead of a map. The array is indexed with indices stored in each variable. This also avoids separately allocating variable states. * Move less data around in the scheduler and use a `Stack` instead of `Map`. `Map` was used before because it allows for some optimizations that might be more important in the future, but they don't matter right now (e.g. joining execution paths that diverged earlier). * Avoid memory allocations by giving the `LinearAllocator` some memory from the stack.