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).

core/src/translation.rs

@@ -0,0 +1,43 @@
+// Copyright 2019 Parity Technologies (UK) Ltd.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+use multiaddr::{Multiaddr, Protocol};
+
+/// Perform IP address translation.
+///
+/// Given an `original` [`Multiaddr`] and some `observed` [`Multiaddr`], return
+/// a translated [`Multiaddr`] which has the first IP address translated by the
+/// corresponding one from `observed`.
+///
+/// This is a mixed-mode translation, i.e. an IPv4 address may be replaced by
+/// an IPv6 address and vice versa.
+///
+/// If the first [`Protocol`]s are not IP addresses, `None` is returned instead.
+pub fn address_translation(original: &Multiaddr, observed: &Multiaddr) -> Option<Multiaddr> {
+    original.replace(0, move |proto| match proto {
+        Protocol::Ip4(_) | Protocol::Ip6(_) => match observed.iter().next() {
+            x@Some(Protocol::Ip4(_)) => x,
+            x@Some(Protocol::Ip6(_)) => x,
+            _ => None
+        }
+        _ => None
+    })
+}
+// TODO: add tests