Previously, the radix iterator unconditionally found the next non-null
leaf node, even if it fell outside of the range the iterator was
derived from.  This was obviously an efficiency issue, but it also
introduced unnecessary sharing because of the reads outside of the
iterator's range.  Now it takes an explicit upper bound and will stop
resolving elements when it reaches that bound.  The end() iterator is
now naturally represented in terms of this bound, rather than in terms
of a special-case key.

For asharing vm, this eliminates all violations of the commutativity
rule.

Since I couldn't figure out how to nicely work this bound into the
existing recursive traversal implementation, I reworked it to be
iterative and, I think, made it much simpler in the process.  This
does make one notable semantic change: previously the radix iterator
cached the element it pointed to, so an update to that element would
not affect the value returned by the iterator.  It no longer caches
it, which makes it behave more like a regular collection iterator.