Shrink the size of Expr (#12610)

# Description
Continuing from #12568, this PR further reduces the size of `Expr` from
64 to 40 bytes. It also reduces `Expression` from 128 to 96 bytes and
`Type` from 32 to 24 bytes.

This was accomplished by:
- for `Expr` with multiple fields (e.g., `Expr::Thing(A, B, C)`),
merging the fields into new AST struct types and then boxing this struct
(e.g. `Expr::Thing(Box<ABC>)`).
- replacing `Vec<T>` with `Box<[T]>` in multiple places. `Expr`s and
`Expression`s should rarely be mutated, if at all, so this optimization
makes sense.

By reducing the size of these types, I didn't notice a large performance
improvement (at least compared to #12568). But this PR does reduce the
memory usage of nushell. My config is somewhat light so I only noticed a
difference of 1.4MiB (38.9MiB vs 37.5MiB).

---------

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

crates/nu-cmd-plugin/src/commands/plugin/list.rs

@@ -13,14 +13,17 @@ impl Command for PluginList {
         Signature::build("plugin list")
             .input_output_type(
                 Type::Nothing,
-                Type::Table(vec![
-                    ("name".into(), Type::String),
-                    ("is_running".into(), Type::Bool),
-                    ("pid".into(), Type::Int),
-                    ("filename".into(), Type::String),
-                    ("shell".into(), Type::String),
-                    ("commands".into(), Type::List(Type::String.into())),
-                ]),
+                Type::Table(
+                    [
+                        ("name".into(), Type::String),
+                        ("is_running".into(), Type::Bool),
+                        ("pid".into(), Type::Int),
+                        ("filename".into(), Type::String),
+                        ("shell".into(), Type::String),
+                        ("commands".into(), Type::List(Type::String.into())),
+                    ]
+                    .into(),
+                ),
             )
             .category(Category::Plugin)
     }