Now, there are three kinds of types used in WhyML API:
  ity -- the type of first-order values, "i" stands for "individual"
  aty -- the type of higher-order values, "a" stands for "arrow"
  vty -- the sum of the previous two, "v" stands for "value"

We should probably rename the VTvalue constructor, since it carries
an ity, and not a vty. And I would gladly rename ity to something
more appropriate, too.

Instead, we now store the ghostness boolean directly
in pvsymbols, pssymbols, ppatterns, and exprs. We thus
remove one level of indirection, and vty_value becomes
a one-field record which we remove in the next commit.

This simplifies semantics, makes a lot of checks unnecessary,
and makes us compatible with OCaml.

the Set signature is moved to a separate Extset module,
instead of being included into Extmap.S. The module hierarchy
becomes simpler, we don't shadow  OCaml standard modules anymore,
and the bug #15270 is fixed, too.

In this way, we can always distinguish them from local theories and
modules. Both 'use Bool' and 'use why3.Bool' are accepted. No support
for use/clone of built-in modules is done yet, but so far we don't
needed (the only built-in module is why3.Prelude which is used by
default).

As of now, one cannot put a file "why3.why" at the root of loadpath,
since it will be inaccessible. Paths like why3/toto.why are still
admitted, but we will probably ban them too and reserve the whole
"why3.xxx.yyy" hierarchy for the built-in theories and modules.

+ rename Mlw_ty.ity_pure to ity_immutable

it is safer to construct the whole pattern from an untyped skeleton
in a top-down way, than to check the ghostness/mutability on each
constructor application.

eval_match expects a well-formed formula, consistent with the known_map