Applied to a term tm of the form `c (\x. t)` where c is a constant whose
name is "s", the call dest_binder "c" tm returns (`x`,`t`). Note that
this is actually independent of whether the name parses as a binder, but the
usual application is where it does.
FAILURE CONDITIONS
Fails if the term is not of the appropriate form with a constant of the same
name.
EXAMPLE
The call dest_binder "!" is the same as dest_forall, and is in fact how
that function is implemented.