Refactor away NestedMetaItemKind

Remove methods `Attribute::span` and `MetaItem::span` duplicating public fields

src/libsyntax_ext/test.rs

@@ -227,7 +227,7 @@ fn should_panic(cx: &ExtCtxt<'_>, i: &ast::Item) -> ShouldPanic {
                         .and_then(|mi| mi.value_str());
                     if list.len() != 1 || msg.is_none() {
                         sd.struct_span_warn(
-                            attr.span(),
+                            attr.span,
                             "argument must be of the form: \
                              `expected = \"error message\"`"
                         ).note("Errors in this attribute were erroneously \