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/field/attribute.rs

@@ -102,7 +102,7 @@ impl Field for AttributeField {
             generator: quote! {
                 #as_optional_xml_text(#bound_name)?.map(|#bound_name| ::xso::Item::Attribute(
                     #xml_namespace,
-                    ::alloc::borrow::Cow::Borrowed(#xml_name),
+                    ::std::borrow::Cow::Borrowed(#xml_name),
                     #bound_name,
                 ));
             },

xso-proc/src/field/child.rs

@@ -423,7 +423,7 @@ impl ExtractDef {
                 quote! {
                     let name = (
                         ::xso::exports::rxml::Namespace::from(#xml_namespace),
-                        ::alloc::borrow::Cow::Borrowed(#xml_name),
+                        ::std::borrow::Cow::Borrowed(#xml_name),
                     );
                     #init
                 }

xso-proc/src/field/text.rs

@@ -51,7 +51,7 @@ impl Field for TextField {
 
         Ok(FieldBuilderPart::Text {
             value: FieldTempInit {
-                init: quote! { ::alloc::string::String::new() },
+                init: quote! { ::std::string::String::new() },
                 ty: string_ty(Span::call_site()),
             },
             collect: quote! {