Throughput has decreased drastically after polling updates

[ryan-summers]

According to @thvdveld and based on observations from @Dirbaio, it was noted that the throughput of Smoltcp has been significantly impacted by #954. Specifically, it appears that this is due to the requirement that egress is checked for every ingressed packet, where this wasn't previously the case.

It was also noted that the current return value of bool from Interface::poll() doesn't provide clear information to a user as to what they should do with that value. We likely want to provide some Enum type instead that is explicit in what the poll result is and what action the user should take. This would future-proof the network stack implementation.