[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/plugins/derive/derive.ml b/plugins/derive/derive.ml
index 1524079f4..6d3d4b743 100644
--- a/plugins/derive/derive.ml
+++ b/plugins/derive/derive.ml
@@ -10,7 +10,7 @@ open Context.Named.Declaration
 
 let map_const_entry_body (f:Term.constr->Term.constr) (x:Safe_typing.private_constants Entries.const_entry_body)
     : Safe_typing.private_constants Entries.const_entry_body =
-  Future.chain ~pure:true x begin fun ((b,ctx),fx) ->
+  Future.chain x begin fun ((b,ctx),fx) ->
     (f b , ctx) , fx
   end