Move in std-embedded-nal

[chrysn]

I've been maintaining std-embedded-nal with assistance from @ryan-summers at https://gitlab.com/chrysn/std-embedded-nal, but given embedded-nal is maintained here and I'd like to get some more community input, I'd like to move that in here.

It doesn't technically fit the criterion of "targets resource constrained devices", but kind'a targets developers of resources constrained devices, for those can use this to test their network applications on unconstrained host systems.

[eldruin]

As you say there, I think it fits the same way we maintain linux-embedded-hal in the REWG so I would welcome it.