Iterable serialization for Rust

[mattiekat]

Is your feature request related to a problem? Please describe.
Sometimes we want to serialize into a Write or AsyncWrite type and to do so without allocating a buffer to store all the data in serialized form before passing it to write requires a way to serialize fields one at a time in a depth-first manner.

Describe the solution you'd like
A new function on Record and SubRecord which returns an Iterator<Item=usize> (saying how many bytes where written on each go) and takes a mutable reference to a [u8;1024] or something buffer which can be used to serialize some data into on each go.

This can either be accomplished with unsafe code (possibly some magic safe code that I can't think of ATM), but we could also pass the buffer to it mutably on the .next(&mut buf) call, but that would mean it no longer conforms to std::iter::Iterator.

We could also use a wrapping struct that is a true Iterator type and only goes 1 byte at a time, and an inner non-iterator type that get's the &mut buf on each call, which only happens when the outer type has exhausted what was written last time.

Describe alternatives you've considered
Write all data into a buffer to then pass to Write types as they can accept them. This requires an extra alloc and is non-ideal.

Additional context
This is useful for RPC and networking in general and would allow us to implement Read for Record types.