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/filesystem/du.rs

@@ -33,7 +33,7 @@ impl Command for Du {
 
     fn signature(&self) -> Signature {
         Signature::build("du")
-            .input_output_types(vec![(Type::Nothing, Type::Table(vec![]))])
+            .input_output_types(vec![(Type::Nothing, Type::table())])
             .allow_variants_without_examples(true)
             .rest(
                 "path",

crates/nu-command/src/filesystem/ls.rs

@@ -45,7 +45,7 @@ impl Command for Ls {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("ls")
-            .input_output_types(vec![(Type::Nothing, Type::Table(vec![]))])
+            .input_output_types(vec![(Type::Nothing, Type::table())])
             // LsGlobPattern is similar to string, it won't auto-expand
             // and we use it to track if the user input is quoted.
             .rest("pattern", SyntaxShape::OneOf(vec![SyntaxShape::GlobPattern, SyntaxShape::String]), "The glob pattern to use.")

crates/nu-command/src/filesystem/watch.rs

@@ -38,7 +38,7 @@ impl Command for Watch {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("watch")
-        .input_output_types(vec![(Type::Nothing, Type::Table(vec![]))])
+        .input_output_types(vec![(Type::Nothing, Type::table())])
             .required("path", SyntaxShape::Filepath, "The path to watch. Can be a file or directory.")
             .required("closure",
             SyntaxShape::Closure(Some(vec![SyntaxShape::String, SyntaxShape::String, SyntaxShape::String])),