Use by-ref compare instead of by-val compare in visit_composite.

When comparing type that has a pointer to itself, `==' may go into infinite recursion.
Comparing by reference avoids this. Since `c' and `ty_compinfo` are Rc<RefCell<..>>,
compare references to interior of RefCell.
Merge pull request #141 from dmitryvk/infinite-recursion-fix

Fix for issue #140