enable and fix stricter clippy linting

- add clippy to .build.yml to run on sr.ht
- add missing docs
- use core:: instead of std:: where possible
- use hypot() and mul_add() for better numeric stability
- make all float tests use asser_approx_eq() and explicitly check for NaN/Inf
- add missing semicolons here and there
- add #[must_use] to ensure caller uses returned value
- add const fn where appropriate
- change visibility from pub(crate) to pub (in private modules)

Signed-off-by: Stephen Waits <steve@waits.net>

.build.yml

@@ -11,13 +11,15 @@ tasks:
   - stable: |
       rustup default stable
       cd vivaldi-nc/
+      cargo clippy --verbose
       cargo build --verbose
       cargo test --verbose
   - nightly: |
       rustup default nightly
       cd vivaldi-nc/
-      cargo build --verbose ||:
-      cargo test --verbose  ||:
+      cargo clippy --verbose ||:
+      cargo build --verbose  ||:
+      cargo test --verbose   ||:
   - docs: |
       cd vivaldi-nc/
       rustup run stable cargo doc --no-deps