Investigate using `VecDeque` to represent the stack in processor

With #2547 we settled with representing stacks conceptually from top-to-bottom/left-to-right. That is, `[a, b, c, d]` means that `a` is on top.

The `FastProcessor` is then the last place in the codebase that uses the reverse ordering (such that `d` would be on top in the previous example). The original reason for doing so was simply so that we could use `Vec::{push,pop}()` to push/pop elements to the stack, which are fast.

We should now investigate using a `VecDeque`, and `{push,pop}_front()` to push/pop elements. If we can confirm that the performance is preserved, then we should switch to that representation, so that we can have the same stack ordering everywhere in our codebase.