Inline prefix into radix tree node

Inline prefix into radix tree node

Embed isc_prefix_t directly in isc_radix_node_t instead of heap-
allocating it separately. This eliminates per-node isc_mem_get/put
and isc_mem_attach/detach calls, removes a pointer dereference on
every search comparison, and simplifies the code by removing the
_new_prefix, _ref_prefix, and _deref_prefix helpers entirely.

Remove isc_mem_t from isc_prefix_t since it is now a plain value
type with no memory management. Remove per-node isc_mem_t since
nodes use the tree's memory context. Reorder struct fields to
eliminate padding holes.