JRFC 31 - multikey

Following in line with multihash, multiaddr, multicodec, and so on, enter multikey.

In a conversation about [ipfs](https://github.com/jbenet/ipfs/) with @whyrusleeping and diffie, future-proofing the system came up. We noted the hashes used multihash. Then we discussed we need to do the same to keys -- in particular for avoiding catastrophe from an unexpected factoring breakthrough.[1]

Tentatively:

```

```

This works for both pub/private keys, as well as symmetric keys. (The algorithm code can signal whether the key is public or private, _say_ 0xc0 for rsa private key, and 0xc1 for corresponding public key)

[1] Another note is to carefully isolate the reliance on PKI, and design systems with transitions to stronger systems (e.g. post-quantum pki systems, like [this](http://eprint.iacr.org/2011/506.pdf)).

No files, tests, or implementation entry points are named. Start by reviewing the multihash, multiaddr, and multicodec context, then examine the proposed varint algorithm, byte length, and key bytes layout. Done means reaching a settled multikey design that covers public, private, and symmetric keys and supports algorithm transitions.

Written by the indexing model from the issue text.