Remove special gid arithmetic ops from cell_member_type. (#685)

Somewhere along the line, some extensions were made to `cell_member_type` to facilitate some arithmetic on the `gid` field. These operations tend to blur the distinction between a gid and a `cell_member_type` value, and are besides used only in a couple of places.

* Remove `cell_member_type` arithmetic with `gid` methods/functions.
* Replace usage of these short cuts with explicit use of `gid` field in `dry_run_context_impl` and `symmetric_recipe`.