xso: fix no_std support

This reverts the Revert in 6ec275d3 which had to be done in order to
un-break usage of xso in std crates.

xso-proc/src/field/attribute.rs

@@ -128,7 +128,7 @@ impl Field for AttributeField {
             generator: quote! {
                 #generator.map(|#bound_name| ::xso::Item::Attribute(
                     #xml_namespace,
-                    ::std::borrow::Cow::Borrowed(#xml_name),
+                    ::xso::exports::alloc::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),
-                        ::std::borrow::Cow::Borrowed(#xml_name),
+                        ::xso::exports::alloc::borrow::Cow::Borrowed(#xml_name),
                     );
                     #init
                 }

xso-proc/src/field/flag.rs

@@ -125,7 +125,7 @@ impl Field for FlagField {
                         2 => ::core::result::Result::Ok(::core::option::Option::Some(
                             ::xso::Item::ElementHeadStart(
                                 ::xso::exports::rxml::Namespace::from(#xml_namespace),
-                                ::std::borrow::Cow::Borrowed(#xml_name),
+                                ::xso::exports::alloc::borrow::Cow::Borrowed(#xml_name),
                             )
                         )),
                         _ => unreachable!(),

xso-proc/src/field/text.rs

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