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.

unless the old proof already starts with "(* intros "

 - use the syntax of the driver for the own realization of a theory
 - add a *_def lemma for defined logics with syntax
 - add comments that inform which syntax is used for declared logic

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.

Otherwise, a goal (A && B) -> C produces axioms A and A -> B
during premise introduction.