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/debug/ast.rs

@@ -16,7 +16,7 @@ impl Command for Ast {
 
     fn signature(&self) -> Signature {
         Signature::build("ast")
-            .input_output_types(vec![(Type::String, Type::Record(vec![]))])
+            .input_output_types(vec![(Type::String, Type::record())])
             .required(
                 "pipeline",
                 SyntaxShape::String,

crates/nu-command/src/debug/info.rs

@@ -31,7 +31,7 @@ impl Command for DebugInfo {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("debug info")
-            .input_output_types(vec![(Type::Nothing, Type::Record(vec![]))])
+            .input_output_types(vec![(Type::Nothing, Type::record())])
             .category(Category::Debug)
     }
 

crates/nu-command/src/debug/metadata.rs

@@ -18,7 +18,7 @@ impl Command for Metadata {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("metadata")
-            .input_output_types(vec![(Type::Any, Type::Record(vec![]))])
+            .input_output_types(vec![(Type::Any, Type::record())])
             .allow_variants_without_examples(true)
             .optional(
                 "expression",

crates/nu-command/src/debug/profile.rs

@@ -34,7 +34,7 @@ impl Command for DebugProfile {
                 "How many blocks/closures deep to step into (default 2)",
                 Some('m'),
             )
-            .input_output_types(vec![(Type::Any, Type::Table(vec![]))])
+            .input_output_types(vec![(Type::Any, Type::table())])
             .category(Category::Debug)
     }
 

crates/nu-command/src/debug/view_files.rs

@@ -20,12 +20,15 @@ impl Command for ViewFiles {
         Signature::build("view files")
             .input_output_types(vec![(
                 Type::Nothing,
-                Type::Table(vec![
-                    ("filename".into(), Type::String),
-                    ("start".into(), Type::Int),
-                    ("end".into(), Type::Int),
-                    ("size".into(), Type::Int),
-                ]),
+                Type::Table(
+                    [
+                        ("filename".into(), Type::String),
+                        ("start".into(), Type::Int),
+                        ("end".into(), Type::Int),
+                        ("size".into(), Type::Int),
+                    ]
+                    .into(),
+                ),
             )])
             .category(Category::Debug)
     }