Register context stacks with Valgrind

The `cps_context_new` function now registers the context's stack with Valgrind, using Valgrind's [client request mechanism](http://www.valgrind.org/docs/manual/manual-core-adv.html#manual-core-adv.clientreq).  This gets rid of some spurious “uninitialized value” warnings that we were getting in code that uses fibers.