Add method to `SlaveGroup` to get all slave statuses at once

[jamwaffles]

This requires internal support for sending multiple PDUs at a time (as many fit in an Ethernet packet) which is a useful feature to have for other purposes.

One option is to add SlaveState::Other(Flags) where Flags is a bitflags struct. This allows both easily checking if every slave in a group is in the same state, as well as collecting different states in some bit flags.