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_plugin_polars/src/dataframe/eager/to_nu.rs

@@ -37,7 +37,7 @@ impl PluginCommand for ToNu {
             .switch("tail", "shows tail rows", Some('t'))
             .input_output_types(vec![
                 (Type::Custom("expression".into()), Type::Any),
-                (Type::Custom("dataframe".into()), Type::Table(vec![])),
+                (Type::Custom("dataframe".into()), Type::table()),
             ])
             .category(Category::Custom("dataframe".into()))
     }