Implement SpkIterator

SpkIterator was created with its own nth() and next() implementations
and its own new() and new_with_range() constructors.

Co-authored-by: 志宇 <hello@evanlinjin.me>

crates/chain/src/lib.rs

@@ -43,6 +43,10 @@ pub use miniscript;
 mod descriptor_ext;
 #[cfg(feature = "miniscript")]
 pub use descriptor_ext::DescriptorExt;
+#[cfg(feature = "miniscript")]
+mod spk_iter;
+#[cfg(feature = "miniscript")]
+pub use spk_iter::*;
 
 #[allow(unused_imports)]
 #[macro_use]