Proto: payer is an implicit account

Context

The payer is always an implicit account, let's make the type system ensure it!

It was wrong in:

• liquidity baking, but the payer is never used by the contract, I turned it to the address zero
• simulations in the client/plugin, I constrained commands/RPCs, and used the address zero as the default value.

There's a bit of peneloping due to pleasing the type system at each commit, the overall change may be as easy to review.

Manually testing the MR

CI

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