Shrink Value by boxing Range/Closure (#12784)

# Description
On 64-bit platforms the current size of `Value` is 56 bytes. The
limiting variants were `Closure` and `Range`. Boxing the two reduces the
size of Value to 48 bytes. This is the minimal size possible with our
current 16-byte `Span` and any 24-byte `Vec` container which we use in
several variants. (Note the extra full 8-bytes necessary for the
discriminant or other smaller values due to the 8-byte alignment of
`usize`)

This is leads to a size reduction of ~15% for `Value` and should overall
be beneficial as both `Range` and `Closure` are rarely used compared to
the primitive types or even our general container types.

# User-Facing Changes
Less memory used, potential runtime benefits.

(Too late in the evening to run the benchmarks myself right now)

crates/nu-cli/src/prompt_update.rs

@@ -65,7 +65,7 @@ fn get_prompt_string(
         .get_env_var(engine_state, prompt)
         .and_then(|v| match v {
             Value::Closure { val, .. } => {
-                let result = ClosureEvalOnce::new(engine_state, stack, val)
+                let result = ClosureEvalOnce::new(engine_state, stack, *val)
                     .run_with_input(PipelineData::Empty);
 
                 trace!(