New compilation mode -vi2vo

To obtain a.vo one can now:
1) coqtop -quick -compile a
2) coqtop -vi2vo a.vi

To make that possible the .vo structure has been complicated.  It is now
made of 5 segments.

               |  vo  | vi  | vi2vo | contents
 --------------+------+-----+-------+------------------------------------
 lib           | Yes  | Yes | Yes   | libstack (modules, notations,...)
 opauqe_univs  | No   | Yes | Yes   | constraints coming from opaque proofs
 discharge     | No   | Yes | No    | data needed to close sections
 tasks         | No   | Yes | No    | STM tasks to produce proof terms
 opaque_proofs | Yes  | Yes | Yes   | proof terms
 --------------+------+-----+-------+------------------------------------

This means one can load only the strictly necessay parts.  Usually one
does not load the tasks segment of a .vi nor the opaque_proof segment of
a .vo, unless one is turning a .vi into a .vo, in which case he load
all the segments.

Optional segments are marshalled as None.  But for lib, all segments
are Array.t of:

               | type
 --------------+---------------------------------------------------------
 lib           | a list of Libobject.obj (n'importe quoi)
 opauqe_univs  | Univ.consraints Future.computation
 discharge     | what Cooking.cook_constr needs
 tasks         | Stm.tasks (a task is system_state * vernacexpr list)
 opaque_proofs | Term.constr Future.computation
 --------------+------+-----+-------+------------------------------------

Invariant: all Future.computation in a vo file (obtained by a vi2vo
compilation or not) have been terminated with Future.join (or
Future.sink).  This means they are values (inside a box).
This invariant does not hold for vi files.  E.g. opauqe_proofs can be
dangling Future.computation (i.e. NotHere exception).  The vi2vo
compilation step will replace them by true values.

Rationale for opaque_univs: in the vi2vo transformation we want to reuse
the lib segment.  Hence the missing pieces have to be put on the side,
not inside.  Opaque proof terms are already in a separte segment.
Universe constraints are not, hence the new opauqe_univs segment.  Such
segment, if present in a .vo file, is always loaded, and
Declare.open_constant will add to the environment the constraints stored
there.  For regular constants this is not necessay since the constraints
are already in their enclosing module (and also in the constant_body).
With vi2vo the constraints coming from the proof are not in the
constant_body (hence not in the enclosing module) but there and are
added to the environment explicitly by Declare.open_constant.

Rationale for discharge: vi2vo produces a proof term in its original
context (in the middle of a section).  Then it has to discharge the
object.  This segment contains the data that is needed in order to do
so.  It is morally the input that Lib.close_section passes to Cooking
(via the insane rewinding of libstack, GlobalRecipe, etc chain).

Checksums: the checksum of .vi and a .vo obtain from it is the same.
This means that if if b.vo has been compiled using a.vi, and then
a.vi is compiled into a.vo, Require Import b works (and recursively
loads a.vo).
checker and votour ported to new vo format (after -vi2vo)
univ: removing dead code
remoteCounter: backup/restore

When you resume the compilation of a .vi file, you want to
avoid collisions on fresh names.
STM: backup/restore remote counters
Lazyconstr -> Opaqueproof

Make this module deal only with opaque proofs.
Make discharging/substitution invariant more explicit via a third constructor.
vi2vo: new flag -schedule-vi2vo
coq_makefile: new target vi2vo
STM: when batch compiling a vo, assert we behave conservatively

This meas that the list of future_constraints in safe_env is empty,
meaning that nothing was delayed.
Library: when compiling multiple files, reset the opaque tables

That was a bug. coqc a b was generating (for b) an opaque table containing
also the proofs of a.
STM: better messages when checking/finishing tasks
refman: document vi2vo