[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, 1 insertions, 1 deletions

diff --git a/vernac/lemmas.ml b/vernac/lemmas.ml
index d45665dd4..9ab89c883 100644
--- a/vernac/lemmas.ml
+++ b/vernac/lemmas.ml
@@ -60,7 +60,7 @@ let adjust_guardness_conditions const = function
   (* Try all combinations... not optimal *)
      let env = Global.env() in
      { const with const_entry_body =
-        Future.chain ~pure:true const.const_entry_body
+        Future.chain const.const_entry_body
         (fun ((body, ctx), eff) ->
           match kind_of_term body with
           | Fix ((nv,0),(_,_,fixdefs as fixdecls)) ->