Add protocols_handler::multi module.

[twittner]

An implementation of ProtocolsHandler that contains multiple other ProtocolsHandlers indexed by some key type.

Factored out from paritytech/substrate#5045.

[mxinden]

For what my review is worth, this looks good to me.

[tomaka]

You also need to provide an implementation of IntoProtocolsHandler.

[twittner]

You also need to provide an implementation of IntoProtocolsHandler.

My understanding was that there is a blanket impl of IntoProtocolsHandler for types that implement ProtocolsHandler.

[tomaka]

My understanding was that there is a blanket impl of IntoProtocolsHandler for types that implement ProtocolsHandler.

All ProtocolsHandlers implement IntoProtocolsHandler, but not the invert.
Right now you can't use a "multi handler" with an H that implements only IntoProtocolsHandler and not ProtocolsHandler.

To rephrase: the "protocols handler" thing is really two steps (the "into", then the actual handler), and for convenience we allow short-circuiting the first step. But in that PR you make it mandatory to short-circuit the first step, which is not great for something generic.