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-protocol/src/value/from_value.rs

@@ -1,4 +1,3 @@
-use std::collections::HashMap;
 use std::path::PathBuf;
 
 use crate::ast::{CellPath, MatchPattern, PathMember};
@@ -496,7 +495,7 @@ impl FromValue for Closure {
             Value::Closure { val, .. } => Ok(val),
             Value::Block { val, .. } => Ok(Closure {
                 block_id: val,
-                captures: HashMap::new(),
+                captures: Vec::new(),
             }),
             v => Err(ShellError::CantConvert {
                 to_type: "Closure".into(),