MODIF: Rename expand_* predicates to rewrite_* predicate, to avoid clashes with
       core prolog predicates with the same names (for instance, those in
       expand.pl)

MODIF: Rename ref_expand.pl to ref_replace.pl and ref_expanders.pl to
       ref_replacers.pl

ADDED: New option linear_term, to read a term from the sources as a linear term,
       and give the variable bindings in a separated list (new mklinear.pl
       module).

FIXED: Bug related to prolog:xref_open_source/2 hook and open_source/2: The hook
       should be enabled only when term_info.pl call it directly, not all time.

ADDED: New tests to check new features/fixes.

FIXED: Being aware of unifications between Into2 and Term3 (predicate
       ref_replace:perform_substitution).