Allow users to use 512-bit network seeds

[urbit]

Allow users to use 512-bit network seeds

[jtobin]

I've just gotten around to looking into this. I believe the rationale here is that the network seed is derived from the 256-bit management seed in the Urbit HD wallet -- the management seed is a BIP39 mnemonic, thus can encode a maximum of 256 bits of entropy.

There's actually no reason we need to enforce 256-bit seeds here, and in fact I think we shouldn't. "Advanced" users, i.e. those not using the HD wallet, should be free to use higher-entropy seeds.

[juped]

Yeah the above all makes sense to me