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/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! {