Reuse Closure type in Value::Closure (#10894)

# Description
Reuses the existing `Closure` type in `Value::Closure`. This will help
with the span refactoring for `Value`. Additionally, this allows us to
more easily box or unbox the `Closure` case should we chose to do so in
the future.

# User-Facing Changes
Breaking API change for `nu_protocol`.

crates/nu-engine/src/env.rs

@@ -387,8 +387,8 @@ fn get_converted_value(
         if let Ok(v) = env_conversions.follow_cell_path_not_from_user_input(path_members, false) {
             let from_span = v.span();
             match v {
-                Value::Closure { val: block_id, .. } => {
-                    let block = engine_state.get_block(block_id);
+                Value::Closure { val, .. } => {
+                    let block = engine_state.get_block(val.block_id);
 
                     if let Some(var) = block.signature.get_positional(0) {
                         let mut stack = stack.gather_captures(engine_state, &block.captures);

crates/nu-engine/src/eval.rs

@@ -5,7 +5,7 @@ use nu_protocol::{
         eval_operator, Argument, Assignment, Bits, Block, Boolean, Call, Comparison, Expr,
         Expression, Math, Operator, PathMember, PipelineElement, Redirection,
     },
-    engine::{EngineState, Stack},
+    engine::{Closure, EngineState, Stack},
     IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, Range, Record, ShellError, Span,
     Spanned, Unit, Value, VarId, ENV_VARIABLE_ID,
 };
@@ -530,7 +530,13 @@ pub fn eval_expression(
             for var_id in &block.captures {
                 captures.insert(*var_id, stack.get_var(*var_id, expr.span)?);
             }
-            Ok(Value::closure(*block_id, captures, expr.span))
+            Ok(Value::closure(
+                Closure {
+                    block_id: *block_id,
+                    captures,
+                },
+                expr.span,
+            ))
         }
         Expr::Block(block_id) => Ok(Value::block(*block_id, expr.span)),
         Expr::List(x) => {