[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/command.ml b/vernac/command.ml
index a1a87d54e0..f095a5d6cf 100644
--- a/vernac/command.ml
+++ b/vernac/command.ml
@@ -79,7 +79,7 @@ let red_constant_entry n ce sigma = function
         let (_, c) = redfun env sigma c in
         EConstr.Unsafe.to_constr c
       in
-      { ce with const_entry_body = Future.chain ~pure:true proof_out
+      { ce with const_entry_body = Future.chain proof_out
         (fun ((body,ctx),eff) -> (under_binders env sigma redfun n body,ctx),eff) }
 
 let warn_implicits_in_term =