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/engine/capture_block.rs

@@ -1,5 +1,3 @@
-use std::collections::HashMap;
-
 use crate::{BlockId, Value, VarId};
 
 use serde::{Deserialize, Serialize};
@@ -7,7 +5,7 @@ use serde::{Deserialize, Serialize};
 #[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct Closure {
     pub block_id: BlockId,
-    pub captures: HashMap<VarId, Value>,
+    pub captures: Vec<(VarId, Value)>,
 }
 
 #[derive(Clone, Debug)]

crates/nu-protocol/src/engine/stack.rs

@@ -138,19 +138,13 @@ impl Stack {
             })
     }
 
-    pub fn captures_to_stack(&self, captures: &HashMap<VarId, Value>) -> Stack {
+    pub fn captures_to_stack(&self, captures: Vec<(VarId, Value)>) -> Stack {
         // FIXME: this is probably slow
         let mut env_vars = self.env_vars.clone();
         env_vars.push(HashMap::new());
 
-        // FIXME make this more efficient
-        let mut vars = vec![];
-        for (id, val) in captures {
-            vars.push((*id, val.clone()));
-        }
-
         Stack {
-            vars,
+            vars: captures,
             env_vars,
             env_hidden: self.env_hidden.clone(),
             active_overlays: self.active_overlays.clone(),