Variant of compute_in_goal that rewrite only the specified set of rules.

Currently, clones "ts -> ty" are represented as "ts -> ts[=ty]",
where the second type symbol is an alias created on the fly
for the desired type. The problem comes when we want to clone
a theory that contains such a cloning declaration. Since these
aliases are "non-citizens" -- they are not properly declared,
they are not put in the known_map, and they are not listed as
locally declared symbols of the theory -- cloning of the cloning
declaration mistakes them for external symbols and ignores them.
Thus, the aliased type goes into the cloned theory as is, which
provokes the error.

The fix consists in an additional test for non-local type symbols.
If such a symbol is an alias for a type that is actually cloneable,
we conclude that the symbol is not external, but is a mere proxy
for the aliased type, and so we clone the type and create a fresh
type alias for it.

A proper fix would be to always clone type symbols into types and
never introduce the proxy type aliases. This is a more intrusive
change, however, so let's start with a bandage.

so as to keep the full path unchanged

new goals are associated to old goals directly in the order they appear.
they are all marked obsolete, unless the theory itself is found non
obsolete (thanks to the new checksums for theories)

in other words, reloading a session on a file that did not change results
in non-obsolete goals, even if checksums and shapes are absent (e.g. if
the file was not put under version control)

and merge the why3 and why3session libraries back into one.

A simple, assembly-like syntax for strategies is introduced.
The code for a strategy is now a single string, in the field
'code' of a 'strategy' entry of a configuration file.
See share/strategies.conf for examples.

check-sums from session in a near future, providing we can compute
check-sum of a while set of files, or of individual file,
or individual sessions (the latter more likely).