Zero-copy network transmission with io_uring

Cows are gassy and slow - even spherical cows in a vacuum.

A language like goland that already has magic in the compiler to promote local variables to the heap automatically where needed might be able to optimise this away behind the scenes (not necessarily mainline golang since it would require baking the semantics of networking into the compiler, which would be odd but a language with similar properties) but it is probably better to provide a less leaky abstraction to programmers. Things done behind the scenes to make one interface look like an older, simpler one are rarely optimal, and this code is about getting the last drop of network performance