All non-memory nodes have a depth of 0

This makes some user code a bit easier: rather than having to know
exactly what sort of nodes are memories (currently just "mem" nodes,
but maybe a "rom" in the future?) when attempting to see if memories
fit, the user can just check for the node's depth.  This works kind of
like width checking: everything will end up with a width, so you can
just check for that rather than manually checking each node type.