test: add another test with various field combinations

This tests build for various combinations of different types, in
particular some involving cloned types in containers like Vec and
Option.

tests/many_fields.rs

@@ -0,0 +1,27 @@
+#[macro_use]
+extern crate derive_into_owned;
+
+use std::borrow::Cow;
+
+#[allow(dead_code, clippy::redundant_allocation)]
+#[derive(IntoOwned, Borrowed)]
+struct TestTypes<'a> {
+    a: Box<Cow<'a, str>>,
+    b: Box<i32>,
+    c: Option<i32>,
+    d: Vec<i32>,
+    e: Option<Box<i32>>,
+    f: Option<Box<Box<i32>>>,
+    g: Box<Option<()>>,
+    h: String,
+    i: (String, String),
+    j: (),
+    k: Option<(i32, String)>,
+    l: Option<(i32, Vec<String>, Cow<'a, str>)>,
+    m: Box<(i32, String, Vec<Cow<'a, str>>)>,
+    n: Vec<(i32, Option<String>, Option<Cow<'a, str>>)>,
+    o: ((), ()),
+    p: (String, (String, (String, String))),
+    #[allow(clippy::type_complexity)]
+    q: (String, (String, (String, Box<Cow<'a, str>>))),
+}