Bound length of strings and arrays in state

[anorth]

A few items in state are strings or byte arrays that are provided from off chain. We should bound the length of these strings to a tight reasonable bound, else we could incur unwanted de/serialization cost as these things are read and written (potentially by parties other than the party controlling the value).

• DealProposal.Label: 100s of bytes?
• MinerInfo.PeerID: 100s of bytes?
• MinerInfo.Multiaddrs: array length of 10s? each value 100s of bytes?
• Multisig.Signers: array length of 100s?

The Address type is limited on CBOR decoding to 64 bytes, though it should also check on encoding.

BigIntegers are limited to 128 bytes, which is probably ok from a serde point of view (though maybe not for arithmetic, #866).

Related to #604

[Stebalien]

#920 currently deals with MinerInfo.PeerID and MinerInfo.Multiaddrs.

[anorth]

@Stebalien done?

[Stebalien]

Not quite. Still need deal label, and signers.

I'm picking 256 for the deal label, same for max signers.