This PR recognizes certain kinds of composite proof terms of the form
`hpre.trans hspec |> (wp prog).mono _ _ hpost` and abstracts them into
bespoke theorems. This should yield smaller proof terms. Sadly, kernel
checking time is unaffected, even regressing a bit. The number of shared
terms stays almost the same (+- a constant). Hence I deactivate the code
path in this patch. We keep the code, though, because it might be useful
in the future, also there are a few other improvements.
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