refactor: Policy satisfier

Remove manual pruning from the policy satisfier. The satisfier still
needs to track the satisfiability of sub-policies, so we introduce a
symmetric Result<T, T> where T is the compiled sub-policy and Ok / Err
signify satisfiability. Refactor the code that satisfies threshold
fragments by using the iterator API and by removing mutable state.

Introduce SatisfierError to better track which errors a satisfier can
actually throw and what to do about them. We can remove the "missing
assembly" error if we allow the construction of hidden nodes.

The satisfier only returns pruned programs and needs to know the
transaction environment because of that. Returning unpruned program
seems like a potential footgun, especially because callers will want to
immediately put their programs on the blockchain.

src/policy/mod.rs

@@ -23,4 +23,4 @@ pub mod sighash;
 pub use ast::Policy;
 pub use error::Error;
 pub use key::{SimplicityKey, ToXOnlyPubkey, Translator};
-pub use satisfy::{Preimage32, Satisfier};
+pub use satisfy::{Preimage32, Satisfier, SatisfierError};