lib: graph: remove useless checks, make functions inline on fast path

lib: graph: remove useless checks, make functions inline on fast path

This patch:

* Removes a few useless checks on the fast path for everything related
  to the graph object.

* Makes the "can consume" flag only written and read in developer mode.

* Makes fast path functions `static inline` within `graph.c`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>