Register context stacks with Valgrind

The `cps_context_new` function now registers the context's stack with
Valgrind, using Valgrind's client request mechanism [1].  This gets rid
of some spurious “uninitialized value” warnings that we were getting in
code that uses fibers.

[1] http://www.valgrind.org/docs/manual/manual-core-adv.html#manual-core-adv.clientreq