Add [Polymorphic] and [Monomorphic] local flag for definitions as well as
[Set Universe Polymorphism] global flag to make all following definitions
polymorphic. Mainly syntax for now.
First part of the big changes to the kernel:
- Const, Ind, Construct now come with a universe level instance
- It is used for type inference in the kernel, which now also takes
a graph as input: actually a set of local universe variables and their
constraints. Type inference just checks that the constraints are enough
to satisfy its own rules.
- Remove polymorphic_arity and _knowing_parameters everywhere: we
don't need full applications for polymorphism to apply anymore, as
we generate fresh variables at each constant/inductive/constructor
application. However knowing_parameters variants might be reinstated
later for optimization.
- New structures exported in univ.mli:
  - universe_list for universe level instances
  - universe_context(_set) for the local universe constraints, also
recording which variables will be local and hence generalized after
inference if defining a polymorphic ind/constant.
- this patch makes coq stop compiling at indtypes.ml
Adapt kernel, library, pretyping, tactics and toplevel to universe polymorphism.
Various degrees of integration, places where I was not sure what to do or
just postponed bigger reorganizations of the code are marked with FIXMEs.
Main changes:
- Kernel now checks constraints and does not infer them anymore.
- The inference functions produce a context of constraints that were checked
during inference, useful to do double-checking of the univ. poly. code
but might be removed later.
- Constant, Inductive entries now have a universe context (local variables
and constraints) associated to them.
- Printing, debugging functions for the new structures are also implemented.
- Now stopping at Logic.v
- Lots of new code in kernel/univ.ml that should be reviewed.
- kernel/indtypes probably does not do what's right when inferring inductive
type constraints.
- Adapted evd to use the new universe context structure.
- Did not deal with unification/evar_conv.
- Add externalisation code for universe level instances.
- Support for polymorphism in pretyping/command and proofs/proofview etc.
  Needed wrapping of [fresh_.._instance] through the evar_map, which
  contains the local state of universes during type-checking.
- Correct the inductive scheme generation to support polymorphism as well.
- Have to review kernel code for correctness, and especially rework the
  computation of universe constraints for inductives.
Stops somewhat later in Logic.v
- Fix naming of local/toplevel universes to be correctly done at typechecking time:
  local variables have no dirpath.
- Add code to do substitution of universes in modules, not finished yet.
- Move fresh_* functions out of kernel, it won't ever build a universe level again!
- Adapt a lot of new_Type to use the correct dirpath and declare the new types in the evar_map
so we keep track of them.
- A bit of code factorization (evd_comb moved, pretype_global).
- Refactor more code
- Adapt plugins code (sometimes wrong, marked with FIXME)
- Fix cases generating unneeded universe (not sure it's ok though)
- Fix scheme generation for good, might have opportunity to cleanup
the terms later.
Init compiles now (which means rewrite, inversion, elim etc.. work as well).
- Unsolved issue of pretyping to lower sorts properly (to Prop for example).
  This has to do with the (Retyping.get_type_of) giving algebraic universes that
  would appear on the right of constraints.
  This makes checking for dangling universes at the end of pretyping fail,
  hence the check in kernel/univ was removed. It should come back when we have
  a fix for this.
- Correctly (?) compute the levels of inductive types.
  Removed old code pertaining to universe polymorphism. Note that we generate
  constraint variables for the conclusion of inductive types invariably.
- Shrink constraints before going to the kernel, combine substitution of the
  smaller universe set with normalization of evars (maybe not done everywhere,
  only ordinary inductives, definitions and proofs)
- More API reworks overall. tclPUSHCONTEXT can be used to add fresh universes
  to the proof goal (used in a few places to get the right instance.
- Quick fix for auto that won't work in the long run. It should always have been
  restricted to take constant references as input, without any loss of generality
  over constrs.
Fix some plugins and insertion of non-polymorphic constants in a module.
Now stops in relation classes.
Cleanup and move code from kernel to library and from pretyping to library too.
Now there is a unique universe counter declared in library/universes.ml along
with all the functions to generate new universes and get fresh constant/inductive
terms.
- Various function renamings
- One important change in kernel/univ.ml: now [sup] can be applied to Prop.
- Adapt records/classes to universe polymorphism
- Now stops in EqDepFacts due to imprecise universe polymorphism.
Forgot to git add those files.
interp_constr returns the universe context

The context is then pushed through the environment (or proof goal
sigma).
- Fix insertion of constants/inductives in env, pushing constraints to
the global env for non-polymorphic ones.
- Add Prop as a universe level to do proper type inference with sorts.
It is allowed to take [sup] of [Prop] now.
- New nf_evar based on new Evd.map(_undefined)
- In proofs/logic.ml: conv_leq_goal might create some constraints that
are now recorded.
- Adapt Program code to universes.
Merge with latest trunk + fixes

-Use new constr_of_global from universes
- fix eqschemes to use polymorphic universes
- begin fixing cctac but f_equal still fails
- fix [simpl] and rest of tacred
- all the eq_constr with mkConst foo should be fixed as well, only
partially done
- Fix term hashing function to recognize equal terms up to universe instances.
- Fix congruence closure to equate terms that differ only in universe instances,
these will be resolved by constraints.
Add a set of undefined universe variables to unification.
Universe variables can now be declared rigid or flexible (unifiable).
Flexible variables are resolved at the end of typechecking by instantiating
them to their glb, adding upper bound constraints associated to them.
Also:
- Add polymorphic flag for inductives.
- Fix cooking partially
- Fix kernel/univ.ml to do normalization of universe expressions at
the end of substitution.
Correct classes/structures universe inference

- Required a bit of extension in Univ to handle Max properly (sup u
(u+1)) was returning (max(u,u+1)) for example.
- Try a version where substitution of universe expressions for universe
levels is allowed at the end of unification. By an invariant this
should only instantiate with max() types that are morally "on the
right" only.
This is controlled using a rigidity attribute of universe variables,
also allowing to properly do unification w.r.t. universes during
typechecking/inference.
- Currently fails in Vectors/Fin.v because case compilation generates
"flexible" universes that actually appear in the term...
Fix unification of universe variables.

- Fix choice of canonical universe in presence of universe constraints,
and do so by relying on a trichotomy for universe variables: rigid
(won't be substituted), flexible (might be if not substituted by an
algebraic) and flexible_alg (always substituted).
- Fix romega code and a few more plugins, most of the standard library
goes through now.
- Had to define some inductives as Polymorphic explicitly to make
proofs go through, more to come, and definitions should be polymorphic
too, otherwise inconsistencies appear quickly (two uses of the same
polymorphic ind through monomorphic functions (like nth on lists of
Props and nats) will fix the monomorphic function's universe with eq
constraints that are incompatible).
- Correct universe polymorphism handling for fixpoint/cofixpoint
definitions.
Fix after rebase with trunk
- Fix romega to use the right universes for list constructors.
- Fix internalization/externalization to deal properly with the
implicit parsing of params.
- Fix fourier tactic w.r.t. GRefs
- Fix substitution saturation of universes.
- Fix number syntax plugin.
- Fix setoid_ring to take its coefficients in a Set rather
than a Type, avoiding a large number of useless universe constraints.
- Fix minor checker decl
- Fix btauto w.r.t. GRef
- Fix proofview to normalize universes in the original types as well.
- Fix definitions of projections to not take two universes at the same level,
but at different levels instead, avoiding unnecessary constraints that could
lower the level of one component depending on the use of the other component.