Update to latest multihash.

[dvc94ch]

Also fixes an unused warning when compiled with no-default-features

[tomaka]

Since the multihash crate doesn't have any changelog:
How does it work for the hasher of a certain multihash to be always available? Does that mean that every single hash algorithm is now included in the crate? Or do we fail to parse unknown multihashes with unsupported algorithms? Or is there a hidden panic?

[dvc94ch]

It hasn't changed, all multihashes have always been included. The only difference is that now digest is called on Code directly instead of heap allocating a hasher, which only makes sense if you want to use the Digest api.

What you could do if you don't want all the hashers is provide a custom Code in libp2p with only the hashes that you want to include. A custom code needs to implement TryFrom<u64> and you can return whatever error you want.

Also multihash would require a feature gate to be added no-default-code.

[tomaka]

Could you add an entry in CHANGELOG.md if you don't mind? Since multihash is exposed on the public API, that's a breaking change that should be mentioned

[dvc94ch]

Added an entry, let me know if you want it to contain more information.

--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,6 +27,8 @@
   be supported. IPv4 listener addresses are not affected by this change.
   [PR 1555](https://github.com/libp2p/rust-libp2p/pull/1555)
 
+- `libp2p-core`: Updated to multihash 0.11.0.
+
 # Version 0.18.1 (2020-04-17)
 
 - `libp2p-swarm`: Make sure inject_dial_failure is called in all situations.