sha3/keccak variants and clarification

[subtly]

Only one sha3 is in the multihash list and it's size cannot be used to infer it's type. sha3-256 can be used and truncated to 128-bits and sha3-512 truncated to 256-bits. Here is a snippet from Mar-2014 NIST document:

The four SHA-3 hash functions are defined from the KECCAK[c] function specified in Sec. 5.2 by appending two bits to the message and by specifying the length of the output, as follows:
SHA3-224(M) = KECCAK[448] (M || 01, 224); 
SHA3-256(M) = KECCAK[512] (M || 01, 256); 
SHA3-384(M) = KECCAK[768] (M || 01, 384); 
SHA3-512(M) = KECCAK[1024] (M || 01, 512).
In each case, the capacity is double the digest length, i.e., c = 2d. The two bits that are appended to the message (i.e., 01) support domain separation; i.e., they distinguish the messages for the SHA-3 hash functions from messages for the SHA-3 XOFs discussed in Sec. 6.2, as well as other domains that may be defined in the future.

[jbenet]

@subtly right, i punted on this until i implemented support for it. thanks for the PR.

[jbenet]

this has been addressed