cext: Memory leak in String and Array handling

When a cext call returns, and it is the last level of invocation (e.g. in a ruby -> cext -> ruby -> cext sequence), then the RString and RArray pointers are reset (set to NULL), but the memory allocated for these values is not freed. Also, since these values are nulled out, when the RubyString or RubyArray object is collected, the memory won't be freed then, either.

This used to be done by a finalizer on an associated RString or RArray java proxy, but it looks like those classes aren't used anymore.