Implementation of guarded_by_destructor is now complete w.r.t. the old kernel:

Implementation of guarded_by_destructor is now complete w.r.t. the old kernel:
 a) bug fixed in eat_lambdas_etc: we cannot apply the term to the residual
    arguments since these are not left arguments
 b) when the recursive argument of the nested "let rec" is safe, the formal
    argument can also be considered safe.

TODO:
This implementation of condition b) could be improved by re-writing eat_lambdas
in such a way that it uses a shift_k function to add the context item to the
"context".