[stm] Remove state-handling from Futures.

We make Vernacentries.interp functional wrt state, and thus remove
state-handling from `Future`. Now, a future needs a closure if it
wants to preserve state.

Consequently, `Vernacentries.interp` takes a state, and returns the
new one.

We don't explicitly thread the state in the STM yet, instead, we
recover the state that was used before and pass it explicitly to
`interp`.

I have tested the commit with the files in interactive, but we aware
that some new bugs may appear or old ones be made more apparent.
However, I am confident that this step will improve our understanding
of bugs.

In some cases, we perform a bit more summary wrapping/unwrapping. This
will go away in future commits; informal timings for a full make:

- master:
  real	2m11,027s
  user	8m30,904s
  sys	1m0,000s

- no_futures:
  real	2m8,474s
  user	8m34,380s
  sys	0m59,156s

1 files changed, 0 insertions, 2 deletions

diff --git a/library/states.ml b/library/states.ml
index 03e4610a6..27e0a94f9 100644
--- a/library/states.ml
+++ b/library/states.ml
@@ -37,5 +37,3 @@ let with_state_protection f x =
   with reraise ->
     let reraise = CErrors.push reraise in
     (unfreeze st; iraise reraise)
-
-let with_state_protection_on_exception = Future.transactify