Avoids things looping forever. Still a lot to fix up: The descend API is
getting messy. We probably want to rewrite the iterator to keep a whole
path, instead of bouncing up and down the tree a lot. Also, a level of
duplicate nodes should be collapsed up a level (we can steal another
bit). The iterator should also be replaced by something more compatible
with crange to avoid the hacks in iteration. But in the meantime, at
least this is functional.

Sigh. Hacks.

Fixes crash when running mapbench.

Rather awkward API, but it's enough to implement skipping of empty
nodes.

unexpected sharing of kernel stack.

Otherwise we eat all the memory trying to allocate a giant complete
radix tree. It still doesn't start as we're iterating over 2^36, but
it's a little better.

caught by asharing/mscan.