Update to proc-macro2 0.3 and tweak apis
diff --git a/src/gen/fold.rs b/src/gen/fold.rs
index 46ac7b4..46e94b2 100644
--- a/src/gen/fold.rs
+++ b/src/gen/fold.rs
@@ -419,7 +419,8 @@
 macro_rules! fold_span_only {
     ($f:ident : $t:ident) => {
         pub fn $f<V: Fold + ?Sized>(_visitor: &mut V, mut _i: $t) -> $t {
-            _i.span = _visitor.fold_span(_i.span);
+            let span = _visitor.fold_span(_i.span());
+            _i.set_span(span);
             _i
         }
     }
@@ -2038,7 +2039,6 @@
 pub fn fold_lit_verbatim<V: Fold + ?Sized>(_visitor: &mut V, _i: LitVerbatim) -> LitVerbatim {
     LitVerbatim {
         token: _i . token,
-        span: _visitor.fold_span(_i . span),
     }
 }
 # [ cfg ( any ( feature = "full" , feature = "derive" ) ) ] # [ cfg ( feature = "full" ) ]