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-command/src/platform/ansi/link.rs

@@ -16,8 +16,8 @@ impl Command for SubCommand {
                     Type::List(Box::new(Type::String)),
                     Type::List(Box::new(Type::String)),
                 ),
-                (Type::Table(vec![]), Type::Table(vec![])),
-                (Type::Record(vec![]), Type::Record(vec![])),
+                (Type::table(), Type::table()),
+                (Type::record(), Type::record()),
             ])
             .named(
                 "text",

crates/nu-command/src/platform/ansi/strip.rs

@@ -26,8 +26,8 @@ impl Command for SubCommand {
             .input_output_types(vec![
                 (Type::String, Type::String),
                 (Type::List(Box::new(Type::String)), Type::List(Box::new(Type::String))),
-                (Type::Table(vec![]), Type::Table(vec![])),
-                (Type::Record(vec![]), Type::Record(vec![])),
+                (Type::table(), Type::table()),
+                (Type::record(), Type::record()),
             ])
             .rest(
                 "cell path",

crates/nu-command/src/platform/input/input_listen.rs

@@ -36,10 +36,10 @@ impl Command for InputListen {
             )
             .input_output_types(vec![(
                 Type::Nothing,
-                Type::Record(vec![
+                Type::Record([
                     ("keycode".to_string(), Type::String),
                     ("modifiers".to_string(), Type::List(Box::new(Type::String))),
-                ]),
+                ].into()),
             )])
     }
 

crates/nu-command/src/platform/term_size.rs

@@ -18,10 +18,7 @@ impl Command for TermSize {
             .category(Category::Platform)
             .input_output_types(vec![(
                 Type::Nothing,
-                Type::Record(vec![
-                    ("columns".into(), Type::Int),
-                    ("rows".into(), Type::Int),
-                ]),
+                Type::Record([("columns".into(), Type::Int), ("rows".into(), Type::Int)].into()),
             )])
     }