Capability of satisfying specific leaf

[miketwenty1]

Right now you can only run the satisfier on the entire taptree. With the Iterator being available to iterate and filter on leafs of interest, I would like to be able to satisfy a chosen specific leaf or set of leaves.

We discussed potentially modifying this function: https://github.com/apoelstra/rust-miniscript/blob/b68bf1354af83f2ebe473ef28b09f461f546a804/src/descriptor/tr.rs#L684

[apoelstra]

I believe the best approach here is to extend the Satisfier API to add a filter_tap_leaf method (name TBD) which returns true/false and can be run on the branch before attempting to satisfy the Miniscript. We would provide a default impl which always returns true (attempt all leaves).

In this line

rust-miniscript/src/descriptor/tr/mod.rs

Line 489 in 1fafde2

for leaf in spend_info.leaves() {

we get a TapTreeIterItem which provides accessors for all sorts of nice stuff (annoyingly, not the control block, which would require using a different iterator thanks to Rust's lack of specialization). We should give that to the Satisfier so it can make a decision.

[apoelstra]

We may also want to offer an API to disable keypath-spending (though this is much easier to do with the existing API since the keypath has a hopefully-unique key in it).