async2sync - convert async FF inputs to sync circuits

    async2sync [options] [selection]

This command replaces async FF inputs with sync circuits emulating the same
behavior for when the async signals are actually synchronized to the clock.

This pass assumes negative hold time for the async FF inputs. For example when
a reset deasserts with the clock edge, then the FF output will still drive the
reset value in the next cycle regardless of the data-in value at the time of
the clock edge.

Currently only $adff cells are supported by this pass.