ALthough transport upgrades must follow a specific pattern
in order fot the resulting transport to be usable with a
`Network` or `Swarm`, that pattern is currently not well
reflected in the transport upgrade API. Rather, transport
upgrades are rather laborious and involve non-trivial code
duplication.

This commit introduces a `transport::upgrade::Builder` that is
obtained from `Transport::upgrade`. The `Builder` encodes the
previously implicit rules for transport upgrades:

  1. Authentication upgrades must happen first.
  2. Any number of upgrades may follow.
  3. A multiplexer upgrade must happen last.

Since multiplexing is the last (regular) transport upgrade (because
that upgrade yields a `StreamMuxer` which is no longer a `AsyncRead`
/ `AsyncWrite` resource, which the upgrade process is based on),
the upgrade starts with `Transport::upgrade` and ends with
`Builder::multiplex`, which drops back down to the `Transport`,
providing a fluent API.

Authentication and multiplexer upgrades must furthermore adhere
to a minimal contract w.r.t their outputs:

  1. An authentication upgrade is given an (async) I/O resource `C`
     and must produce a pair `(I, D)` where `I: ConnectionInfo` and
     `D` is a new (async) I/O resource `D`.
  2. A multiplexer upgrade is given an (async) I/O resource `C`
     and must produce a `M: StreamMuxer`.

To that end, two changes to the `secio` and `noise` protocols have been
made:

  1. The `secio` upgrade now outputs a pair of `(PeerId, SecioOutput)`.
     The former implements `ConnectionInfo` and the latter `AsyncRead` /
     `AsyncWrite`, fulfilling the `Builder` contract.

  2. A new `NoiseAuthenticated` upgrade has been added that wraps around
     any noise upgrade (i.e. `NoiseConfig`) and has an output of
     `(PeerId, NoiseOutput)`, i.e. it checks if the `RemoteIdentity` from
     the handshake output is an `IdentityKey`, failing if that is not the
     case. This is the standard upgrade procedure one wants for integrating
     noise with libp2p-core/swarm.