Code clean-up.

* mh/notes-cleanup:
  load_subtree(): check that `prefix_len` is in the expected range
  load_subtree(): declare some variables to be `size_t`
  hex_to_bytes(): simpler replacement for `get_oid_hex_segment()`
  get_oid_hex_segment(): don't pad the rest of `oid`
  load_subtree(): combine some common code
  get_oid_hex_segment(): return 0 on success
  load_subtree(): only consider blobs to be potential notes
  load_subtree(): check earlier whether an internal node is a tree entry
  load_subtree(): separate logic for internal vs. terminal entries
  load_subtree(): fix incorrect comment
  load_subtree(): reduce the scope of some local variables
  load_subtree(): remove unnecessary conditional
  notes: make GET_NIBBLE macro more robust