Implement bincode Writer and Reader traits for Vec

I am working on a no_std embedded comms stack which talks to devices running C applications. The stack uses heapless::Vecs to handle the byte streams being sent and received, and I would like to use bincode to perform the conversion to / from Rust structures.

Right now, in order to do this I have to create a wrapper type around the heapless::Vec in order to implement bincode's Writer and Reader traits. It seems like this is a common enough pairing that it would make sense to have these traits implemented for heapless::Vec by default, or at least available through a feature, rather than having to re-implement for each new project working in this space.

I'm not sure whether it would make more sense for this implementation to live in the bincode crate or the heapless crate, so I've created an equivalent issue on the bincode repo. What do you think?

Start with the equivalent bincode issue linked in this report and inspect how heapless::Vec is exposed for no_std use. Resolve whether the Writer and Reader implementations belong in heapless or bincode, and whether they need a feature; done means users can use heapless::Vec directly without a wrapper.