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/system/complete.rs

@@ -13,7 +13,7 @@ impl Command for Complete {
     fn signature(&self) -> Signature {
         Signature::build("complete")
             .category(Category::System)
-            .input_output_types(vec![(Type::Any, Type::Record(vec![]))])
+            .input_output_types(vec![(Type::Any, Type::record())])
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/system/ps.rs

@@ -23,7 +23,7 @@ impl Command for Ps {
 
     fn signature(&self) -> Signature {
         Signature::build("ps")
-            .input_output_types(vec![(Type::Nothing, Type::Table(vec![]))])
+            .input_output_types(vec![(Type::Nothing, Type::table())])
             .switch(
                 "long",
                 "list all available columns for each entry",

crates/nu-command/src/system/sys.rs

@@ -18,7 +18,7 @@ impl Command for Sys {
         Signature::build("sys")
             .filter()
             .category(Category::System)
-            .input_output_types(vec![(Type::Nothing, Type::Record(vec![]))])
+            .input_output_types(vec![(Type::Nothing, Type::record())])
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/system/uname.rs

@@ -16,7 +16,7 @@ impl Command for UName {
 
     fn signature(&self) -> Signature {
         Signature::build("uname")
-            .input_output_types(vec![(Type::Nothing, Type::Table(vec![]))])
+            .input_output_types(vec![(Type::Nothing, Type::table())])
             .category(Category::System)
     }
 

crates/nu-command/src/system/which_.rs

@@ -12,7 +12,7 @@ impl Command for Which {
 
     fn signature(&self) -> Signature {
         Signature::build("which")
-            .input_output_types(vec![(Type::Nothing, Type::Table(vec![]))])
+            .input_output_types(vec![(Type::Nothing, Type::table())])
             .allow_variants_without_examples(true)
             .required("application", SyntaxShape::String, "Application.")
             .rest("rest", SyntaxShape::String, "Additional applications.")