Some algorithms don't implement Sum

[coryschwartz]

These algorighms are found in multiformats.Names, but are never registered in sum.go. As a result, attempting to use multiformats.Sum with these algorighms raises ErrSumNotSupported.

I have no need for these, myself, but it seems inconsistent for Sum to work on some algorithms and not others. Is there any reason these are not registered?

keccak-224
x11
keccak-384

[coryschwartz]

I'd like to take a stab at this PR -- do let me know if something needs changed here.

x11 was added last year, it seems just not registered at the time.

As for the keccac functions I could imagine these just being aliased to the sha3 variant, although that's not quite right. Maybe the variants that lack a NewLegacyKeccak* method can just be removed -- I am not sure.

[Stebalien]

Hm. I'm not sure what to do about these.

• We added a definition for the x11 code to this package but never intended to implement it in this package (too many additional dependencies).
• We removed support for keccak-224 and keccak-384 to remove an external dependency.

My concern is that these hashes shouldn't be rejected as invalid just because this implementation of multihash doesn't implement them.

We should consider simply removing the ValidCode check and registering the code in the Names and Codes maps inside RegisterHashFunc. But this could have other implications (some error cases will no longer be caught) so I don't want to rush any decisions here.