fix#3
This takes all Valgrind functionality private again.
Valgrind stack registrations are now associated with a Context, not with
a Stack. This makes sense, since it's only actually a stack when a
Context is running on it. Perhaps Valgrind will even be able to detect
early stack frees now.
We're now handling more of this ourselves, and leaving less to libstd.
Hopefully, we'll eventually break free of libstd, leaving a
highly-focused low-level library that retains all its conveniences in
freestanding environments.
edef