fix(expr)!: revert recursive Cow<[Expression]> back to Vec

This fails to build on 1.65:

```
error[E0277]: the trait bound `[SetItem]: ToOwned` is not satisfied in `Expression`
   --> src/stmt.rs:245:14
    |
245 |     pub dev: Option<Expression>,
    |              ^^^^^^^^^^^^^^^^^^ within `Expression`, the trait `ToOwned` is not implemented for `[SetItem]`, which is required by `Expression: Sized`
    |
    = help: the trait `ToOwned` is implemented for `[T]`
note: required because it appears within the type `Expression`
   --> src/expr.rs:10:10
    |
10  | pub enum Expression {
    |          ^^^^^^^^^^
note: required by an implicit `Sized` bound in `std::option::Option`
   --> /rustc/9b00956e56009bab2aa15d7bff10916599e3d6d6/library/core/src/option.rs:572:1

...and many, many similar errors
```

It compiles successfully on Rust >=1.79, so this commit can be reverted once our MSRV reaches it.

src/expr.rs

@@ -13,7 +13,7 @@ pub enum Expression {
     Number(u32),
     Boolean(bool),
     /// List expressions are constructed by plain arrays containing of an arbitrary number of expressions.
-    List(Cow<'static, [Expression]>),
+    List(Vec<Expression>),
     BinaryOperation(Box<BinaryOperation>),
     Range(Box<Range>),
 
@@ -26,10 +26,10 @@ pub enum Expression {
 /// Wrapper for non-immediate `Expression`s.
 pub enum NamedExpression {
     /// Concatenate several expressions.
-    Concat(Cow<'static, [Expression]>),
+    Concat(Vec<Expression>),
     /// This object constructs an anonymous set.
     /// For mappings, an array of arrays with exactly two elements is expected.
-    Set(Cow<'static, [SetItem]>),
+    Set(Vec<SetItem>),
     Map(Box<Map>),
     Prefix(Prefix),
 

tests/serialize.rs

@@ -4,7 +4,7 @@ use std::borrow::Cow;
 #[test]
 fn test_serialize() {
     let _a: Nftables = Nftables {
-        objects: Cow::Borrowed(&[
+        objects: Cow::Owned(vec![
             NfObject::CmdObject(NfCmd::Add(NfListObject::Table(Table {
                 family: NfFamily::INet,
                 name: Cow::Borrowed("namib"),
@@ -26,12 +26,12 @@ fn test_serialize() {
                 family: NfFamily::INet,
                 table: Cow::Borrowed("namib"),
                 chain: Cow::Borrowed("one_chain"),
-                expr: Cow::Borrowed(&[
+                expr: Cow::Owned(vec![
                     Statement::Match(Match {
-                        left: Expression::List(Cow::Borrowed(&[
+                        left: Expression::List(vec![
                             Expression::Number(123),
                             Expression::String(Cow::Borrowed("asd")),
-                        ])),
+                        ]),
                         right: Expression::Named(NamedExpression::CT(CT {
                             key: Cow::Borrowed("state"),
                             family: None,