Use Vec for Closure captures (#10940)

# Description
Changes the `captures` field in `Closure` from a `HashMap` to a `Vec`
and makes `Stack::captures_to_stack` take an owned `Vec` instead of a
borrowed `HashMap`.

This eliminates the conversion to a `Vec` inside `captures_to_stack` and
makes it possible to avoid clones altogether when using an owned
`Closure` (which is the case for most commands). Additionally, using a
`Vec` reduces the size of `Value` by 8 bytes (down to 72).

# User-Facing Changes
Breaking API change for `nu-protocol`.

crates/nu-command/src/charting/hashable_value.rs

@@ -182,7 +182,7 @@ mod test {
         ast::{CellPath, PathMember},
         engine::Closure,
     };
-    use std::collections::{HashMap, HashSet};
+    use std::collections::HashSet;
 
     #[test]
     fn from_value() {
@@ -243,7 +243,7 @@ mod test {
             Value::closure(
                 Closure {
                     block_id: 0,
-                    captures: HashMap::new(),
+                    captures: Vec::new(),
                 },
                 span,
             ),