Remove some unnecessary static Vecs (#11947)

Avoid unnecessary allocations or larger iterator structs

- Turn static `Vec`s into arrays when possible
- Use `std::iter::once`/`empty` where applicable
- Use `bool::then_some` in `detect column` `.chain`
- Drop in the bucket: de-vec-ing tests

crates/nu-command/src/date/utils.rs

@@ -46,7 +46,7 @@ pub(crate) fn generate_strftime_list(head: Span, show_parse_only_formats: bool)
         description: &'a str,
     }
 
-    let specifications = vec![
+    let specifications = [
         FormatSpecification {
             spec: "%Y",
             description: "The full proleptic Gregorian year, zero-padded to 4 digits.",