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_formats/src/from/eml.rs

@@ -24,7 +24,7 @@ impl SimplePluginCommand for FromEml {
 
     fn signature(&self) -> Signature {
         Signature::build(self.name())
-            .input_output_types(vec![(Type::String, Type::Record(vec![]))])
+            .input_output_types(vec![(Type::String, Type::record())])
             .named(
                 "preview-body",
                 SyntaxShape::Int,

crates/nu_plugin_formats/src/from/ics.rs

@@ -23,7 +23,7 @@ impl SimplePluginCommand for FromIcs {
 
     fn signature(&self) -> Signature {
         Signature::build(self.name())
-            .input_output_types(vec![(Type::String, Type::Table(vec![]))])
+            .input_output_types(vec![(Type::String, Type::table())])
             .category(Category::Formats)
     }
 

crates/nu_plugin_formats/src/from/ini.rs

@@ -20,7 +20,7 @@ impl SimplePluginCommand for FromIni {
 
     fn signature(&self) -> Signature {
         Signature::build(self.name())
-            .input_output_types(vec![(Type::String, Type::Record(vec![]))])
+            .input_output_types(vec![(Type::String, Type::record())])
             .category(Category::Formats)
     }
 

crates/nu_plugin_formats/src/from/vcf.rs

@@ -22,7 +22,7 @@ impl SimplePluginCommand for FromVcf {
 
     fn signature(&self) -> Signature {
         Signature::build(self.name())
-            .input_output_types(vec![(Type::String, Type::Table(vec![]))])
+            .input_output_types(vec![(Type::String, Type::table())])
             .category(Category::Formats)
     }