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-cli/src/commands/keybindings_default.rs

@@ -12,7 +12,7 @@ impl Command for KeybindingsDefault {
     fn signature(&self) -> Signature {
         Signature::build(self.name())
             .category(Category::Platform)
-            .input_output_types(vec![(Type::Nothing, Type::Table(vec![]))])
+            .input_output_types(vec![(Type::Nothing, Type::table())])
     }
 
     fn usage(&self) -> &str {

crates/nu-cli/src/commands/keybindings_list.rs

@@ -14,7 +14,7 @@ impl Command for KeybindingsList {
 
     fn signature(&self) -> Signature {
         Signature::build(self.name())
-            .input_output_types(vec![(Type::Nothing, Type::Table(vec![]))])
+            .input_output_types(vec![(Type::Nothing, Type::table())])
             .switch("modifiers", "list of modifiers", Some('m'))
             .switch("keycodes", "list of keycodes", Some('k'))
             .switch("modes", "list of edit modes", Some('o'))

crates/nu-cli/src/syntax_highlight.rs

@@ -361,7 +361,7 @@ fn find_matching_block_end_in_expr(
             Expr::Nothing => None,
             Expr::Garbage => None,
 
-            Expr::Table(hdr, rows) => {
+            Expr::Table(table) => {
                 if expr_last == global_cursor_offset {
                     // cursor is at table end
                     Some(expr_first)
@@ -370,11 +370,11 @@ fn find_matching_block_end_in_expr(
                     Some(expr_last)
                 } else {
                     // cursor is inside table
-                    for inner_expr in hdr {
+                    for inner_expr in table.columns.as_ref() {
                         find_in_expr_or_continue!(inner_expr);
                     }
-                    for row in rows {
-                        for inner_expr in row {
+                    for row in table.rows.as_ref() {
+                        for inner_expr in row.as_ref() {
                             find_in_expr_or_continue!(inner_expr);
                         }
                     }