Remove Transport::nat_traversal and refactor multiaddr. (#1052)

The functionality is available through `Multiaddr::replace`.
What we currently call "nat_traversal" is merley a replacement of an IP
address prefix in a `Multiaddr`, hence it can be done directly on
`Multiaddr` values instead of having to go through a `Transport`.

In addition this PR consolidates changes made to `Multiaddr` in
previous commits which resulted in lots of deprecations. It adds some
more (see below for the complete list of API changes) and removes all
deprecated functionality, requiring a minor version bump.

Here are the changes to `multiaddr` compared to the currently published
version:

1.  Removed `into_bytes` (use `to_vec` instead).
2.  Renamed `to_bytes` to `to_vec`.
3.  Removed `from_bytes` (use the `TryFrom` impl instead).
4.  Added `with_capacity`.
5.  Added `len`.
6.  Removed `as_slice` (use `AsRef` impl instead).
7.  Removed `encapsulate` (use `push` or `with` instead).
8.  Removed `decapsulate` (use `pop` instead).
9.  Renamed `append` to `push`.
10. Added `with`.
11. Added `replace`.
12. Removed `ToMultiaddr` trait (use `TryFrom` instead).

src/bandwidth.rs

@@ -78,10 +78,6 @@ where
             .dial(addr)
             .map(move |fut| BandwidthFuture { inner: fut, sinks })
     }
-
-    fn nat_traversal(&self, server: &Multiaddr, observed: &Multiaddr) -> Option<Multiaddr> {
-        self.inner.nat_traversal(server, observed)
-    }
 }
 
 /// Wraps around a `Stream` that produces connections. Wraps each connection around a bandwidth