ci: dedicated profile for release + debug_assert

[conorsch]

We want to use the --release behavior, but doing so directly would disable evaluation of debug_assert lines, so we'll selectively override that one option.

Updates the "check" and "test" jobs to use the --profile ci. Also ensures that "check" covers all targets, including benchmarks, and accordingly cleans up an unused import in same.

[conorsch]

To ensure this change does what we want, I applied this diff locally:

index c4f1f58..c25c8ef 100644
--- a/src/elligator.rs
+++ b/src/elligator.rs
@@ -52,6 +52,8 @@ impl Element {
             inner: EdwardsProjective::new(E * H, F * G, E * G, F * H),
         };

+        // Intentional panic to verify this line works in tests
+        debug_assert!(false);
         debug_assert!(
             result.inner.is_on_curve(),
             "resulting point must be on the curve",

and confirmed that:

• cargo nextest run --profile ci --cargo-profile release passed, because debug_asserts are dropped
• cargo nextest run --profile ci --cargo-profile ci failed, due to the debug_assert(false) being evaluated

That's good enough for me.