hmap: Rename hmap_moved() to hmap_node_moved().

hmap: Rename hmap_moved() to hmap_node_moved().

This prepares for adding a new function that deals with a "struct hmap"
moving, as opposed to a "struct hmap_node".

Since there was only a single call to this in the whole tree, and its
caller didn't have any callers of its own at all, also move this function
from hmap.h to hmap.c.