Don't leak thread stack on exit

[tbu-]

Current implementation does not bother with freeing the child thread's resources.

[japaric]

Isn't this equivalent to #134? You need to implement Drop for Thread to not leak the stack, right?

[tbu-]

No, this is orthogonal. Drop on the thread handle should detach the thread (whatever that means) and destructing thread-local data must happen on thread exit.