Resolve some default-warn clippy lints in the geozero-shp crate

[rye]

Some of the code of the geozero-shp crate was copied over from shapefile-rs in 08ef12816e3f3aa2b4a5b148b22927f1399cd0c6. As-is, that code has some clippy warnings in the clippy::all set. This makes hacking on the geozero-shp crate a bit cumbersome, as it's hard to just call cargo clippy and know whether your changes introduced new clippy lints.

The changes in this PR seem to — for the most part, at least — apply to those parts that were copied over. I didn't individually git-blame to verify this, but I didn't encounter anything geozero-specific in the changes I made.

I also intentionally kept this PR very sparse (only making minimal changes so as to resolve clippy lints). I aim to follow up with another PR that does more cleanup, hopefully to include some more intense refactoring and full documentation.

[rye]

A couple of addenda:

• I am happy to add #![deny(clippy::all)] to geozero-shp/src/lib.rs to make the "default Rust style" strictly-enforced through errors.
• I didn't touch the other crates in the workspace. I am happy to do this as well!
  • I would even add cargo clippy to the CI workflow if that would be helpful.

[pka]

Thanks a lot! I'm fine with any clippy fix and a CI workflow would be helpful, indeed!