chore(clippy): enable indexing_slicing and unwrap_in_result lints

`clippy::indexing_slicing` flags any `&v[i]` / `&v[a..b]` that could
panic at runtime, complementing the existing `deny(clippy::panic)`
which does not see slice-index panics from transitive crates.
`clippy::unwrap_in_result` flags `.unwrap()` / `.expect()` inside
functions that return `Result`, where bubbling the error is almost
always preferable.

Enable both for non-test code; tests retain the latitude they already
have via the existing cfg_attr pattern.

This commit is intentionally lint-failing: subsequent commits in this
PR fix the call sites that the new lints surface.

Author: AlfioEmanueleFresta

libwebauthn/src/lib.rs

@@ -5,6 +5,8 @@
 #![cfg_attr(not(test), deny(clippy::panic))]
 #![cfg_attr(not(test), deny(clippy::todo))]
 #![cfg_attr(not(test), deny(clippy::unreachable))]
+#![cfg_attr(not(test), deny(clippy::indexing_slicing))]
+#![cfg_attr(not(test), deny(clippy::unwrap_in_result))]
 
 pub mod fido;
 pub mod management;