[wiggle] Don't make generated structs and unions Copy.

Some structs and unions are large enough that making them `Copy` isn't
ideal. wasi-common only needed `Copy` in a few places that were easy to
fix. `SubscriptionClock` is 32 bytes, so it's not a bad a idea to pass
it by reference anyway.

crates/wiggle/crates/generate/src/types/struct.rs

@@ -74,7 +74,7 @@ pub(super) fn define_struct(
     let (struct_lifetime, extra_derive) = if s.needs_lifetime() {
         (quote!(<'a>), quote!())
     } else {
-        (quote!(), quote!(, Copy, PartialEq))
+        (quote!(), quote!(, PartialEq))
     };
 
     let transparent = if s.is_transparent() {

crates/wiggle/crates/generate/src/types/union.rs

@@ -68,7 +68,7 @@ pub(super) fn define_union(names: &Names, name: &witx::Id, u: &witx::UnionDataty
     let (enum_lifetime, extra_derive) = if u.needs_lifetime() {
         (quote!(<'a>), quote!())
     } else {
-        (quote!(), quote!(, Copy, PartialEq))
+        (quote!(), quote!(, PartialEq))
     };
 
     quote! {