we still keep bv_vars in the binders, so calculating the set
of free variables only has to descend to the topmost binders.
The difference on an example from BWare is quite striking:

/usr/bin/time why3-replayer : with t_vars

505.14user 15.58system 8:40.45elapsed 100%CPU (0avgtext+0avgdata 3140336maxresident)k

/usr/bin/time why3-replayer : without t_vars

242.96user 12.04system 4:16.31elapsed 99%CPU (0avgtext+0avgdata 2007184maxresident)k

Not only we take 2/3 of memory, but we also gain in speed (less work
for the GC, most probably).

This patch should be tested on big WhyML examples,
since src/whyml/mlw_*.ml are big users of t_vars.

Thanks to Guillaume for the suggestion.