fixups to Tr descriptor code from SarcasticNastik

Co-authored-by: SarcasticNastik <aman.rojjha@research.iiit.ac.in>

Fix TapTree iter depth

Fix tr spend_info lock
* Attempt to read cache prior to matching on spend_info Option
* This bug would cause function to hang
* Also added test tr_script_pubkey

Co-authored-by: nickfarrow <nicholas.w.farrow@gmail.com>

There are multiple `Tree`s in this file and the expression Tree is a
somewhat obscure one. It is confusing to have it referenced without
any qualification.

1. `RwLock` is typically much slower than `Mutex`, so the Rust developers
   discourage its use unless you really need lots of simultaneous readers.
   We do not, we only need to unlock this mutex for tiny amounts of time locking logic

2. The `clone` implementation missed an opportunity to clone an internal
   `Arc`, instead using `Mutex::new(None)` and forcing the clone to recompute
   the spend info.

3. Minor idiomatic cleanups.