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.

generic theories can be moved to theories/list.why

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

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.