Revert "xso-proc: Replace std stuff with alloc/core stuff"

This reverts commit 4e5f0bc961.

Unfortunately, in std contexts, the `alloc` crate is not imported. That
means we cannot rely on it being accessible and it in fact breaks builds
of crates which are not `no_std`.

Fixes #155.

xso-proc/src/structs.rs

@@ -294,7 +294,7 @@ impl StructInner {
                     quote! {
                         let name = (
                             ::xso::exports::rxml::Namespace::from(#xml_namespace),
-                            ::alloc::borrow::Cow::Borrowed(#xml_name),
+                            ::std::borrow::Cow::Borrowed(#xml_name),
                         );
                         #init
                     }