net: reconsider buffer allocation

[Dirbaio]

Currently net is responsible for allocating buffers from a PacketPool.

• It doesn't play nicely at all with drivers with special needs for DMA, like
  • special alignment
  • special RAM region so DMA can read it
  • extra prepended data such as DMA descriptors
• It is possible for drivers to hold on to packets for too long, which can make the pool full.
• When it gets full, core can go into a hot poll loop of "try to allocate, fail, retry".
• It requires this Poor Man's Vec that's "data + range".

Maybe allocation should be handled by the driver, like in smoltcp...