staking: Add a burn address

[Yawning]

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While there is an explicit burn transaction type, there should also probably be a burn address that has identical semantics so that burns are more accessible to do.

Low priority since as of now, it is likely the only thing that has used the burn tx is unit tests.

[Yawning]

There is a downside to this in that, if the consensus layer native token ever ends up becoming a meme shitcoin a la SafeLunaInu or whatever, you will end up with people posting personal addresses as the "burn" address on reddit. But that's both amusing and the price to pay for having a more accessible burn mechanism.

Why Terra uses a burn address rather than a burn tx when their entire userbase basically uses their (rather nice) wallet is beyond me.