Clippy fixes from stable and nightly (#13455)

- **Doccomment style fixes**
- **Forgotten stuff in `nu-pretty-hex`**
- **Don't `for` around an `Option`**
- and more

I think the suggestions here are a net positive, some of the suggestions
moved into #13498 feel somewhat arbitrary, I also raised
https://github.com/rust-lang/rust-clippy/issues/13188 as the nightly
`byte_char_slices` would require either a global allow or otherwise a
ton of granular allows or possibly confusing bytestring literals.

crates/nu-command/tests/commands/random/mod.rs

@@ -3,5 +3,4 @@ mod chars;
 mod dice;
 mod float;
 mod int;
-#[cfg(feature = "uuid_crate")]
 mod uuid;

crates/nu-command/tests/commands/random/uuid.rs

@@ -1,5 +1,5 @@
 use nu_test_support::nu;
-use uuid_crate::Uuid;
+use uuid::Uuid;
 
 #[test]
 fn generates_valid_uuid4() {