Use a boxed slice for Export struct (#5887)

## Summary

The vector of names here is immutable -- we never push to it after
initialization. Boxing reduces the size of the variant from 32 bytes to
24 bytes. (See:
https://nnethercote.github.io/perf-book/type-sizes.html#boxed-slices.)
It doesn't make a difference here, since it's not the largest variant,
but it still seems like a prudent change (and I was considering adding
another field to this variant, though I may no longer do so).

crates/ruff_python_semantic/src/binding.rs

@@ -283,7 +283,7 @@ impl<'a> FromIterator<Binding<'a>> for Bindings<'a> {
 #[derive(Debug, Clone)]
 pub struct Export<'a> {
     /// The names of the bindings exported via `__all__`.
-    pub names: Vec<&'a str>,
+    pub names: Box<[&'a str]>,
 }
 
 /// A binding for an `import`, keyed on the name to which the import is bound.