policy: allow exact annex padding for simplicity spends

[delta1]

Implements BlockstreamResearch/simplicity#290 for Elements standardness policy.

Looking for approach feedback.

90c380f should probably be upstreamed into simplicity itself?

Something concerning to be investigated is that Cost::get_padding in rust-simplicity is returning a padding 3 bytes bigger than this calculation.. presumably an error in that method?

[roconnor-blockstream]

I'll have some more comment later but first and foremost there is no need to amend the Simplicity C code.

Instead there is a minCost parameter that can needs to be passed up through CheckSimplicity. See https://github.com/roconnor-blockstream/bitcoin/pull/1/changes#diff-a0337ffd7259e8c7c9a7786d6dbd420c80abfa1afdb34ebae3261109d9ae3c19R1876-R1886 for an example of this. You will need to pass a minCost of 0 when in consensus mode rather than policy mode.

Have a careful read of the documentation for minCost in execSimplicity.