Lifetime elision warnings in iterator methods

[BRIGHTON-ASUMANI]

Fix lifetime elision warnings in iterator methods

Description

The codebase currently has 8 compiler warnings about "hiding a lifetime that's elided elsewhere is confusing" in various iterator method signatures. These warnings make the code less clear and clutter the compiler output.

Current Behavior

When running cargo test --all-targets --all-features, the following warnings appear:

warning: hiding a lifetime that's elided elsewhere is confusing
   --> src/descriptor/mod.rs:291:26
    |
291 |     pub fn tap_tree_iter(&self) -> tr::TapTreeIter<Pk> {
    |                          ^^^^^     ------------------- the same lifetime is hidden here
    |                          |
    |                          the lifetime is elided here
    |
    = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
    = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: use `'_` for type paths
    |
291 |     pub fn tap_tree_iter(&self) -> tr::TapTreeIter<'_, Pk> {
    |                                                    +++

Similar warnings appear in:

• src/descriptor/tr/mod.rs
• src/descriptor/tr/spend_info.rs
• src/descriptor/tr/taptree.rs
• src/miniscript/iter.rs
• src/policy/concrete.rs
• src/primitives/threshold.rs

Expected Behavior

All iterator method signatures should use explicit lifetime parameters ('_) to make the code more clear and eliminate compiler warnings.

Impact

• Low risk: Purely cosmetic changes that don't affect functionality
• High benefit: Eliminates 8 compiler warnings and improves code clarity
• No breaking changes: All existing APIs remain unchanged
• Follows Rust best practices: Makes lifetime relationships explicit

Proposed Solution

Add explicit '_ lifetime parameters to the following method signatures:

// Before
pub fn tap_tree_iter(&self) -> tr::TapTreeIter<Pk> {

// After  
pub fn tap_tree_iter(&self) -> tr::TapTreeIter<'_, Pk> {

This change should be applied to all 8 affected methods across the 7 files mentioned above.

Additional Context

This is a straightforward fix that improves code quality without any functional changes. All tests continue to pass after the changes, confirming no regressions are introduced.

[apoelstra]

Please use some judgement before pasting a million lines of LLM slop into issues.

The actual issue here is "when you run clippy there are new warnings". This has been signalled by #805 for a while and fixed in #849. Nobody cares to read the "Impact" and "Additonal Context" and "Expected Behavior". Have some respect.

[BRIGHTON-ASUMANI]

Please use some judgement before pasting a million lines of LLM slop into issues.

The actual issue here is "when you run clippy there are new warnings". This has been signalled by #805 for a while and fixed in #849. Nobody cares to read the "Impact" and "Additonal Context" and "Expected Behavior". Have some respect.

Thanks for the feedback @apoelstra