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.
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