proc: auto implement Context on command target

Context and TryFromDsl overlap

output/src/view.rs

@@ -103,7 +103,7 @@ pub trait ViewContext<'state, E: Output + 'state>: Send + Sync
         > TryFromDsl<$lt_state, T> for $Struct {
             fn try_from_expr <$lt_source: $lt_state> (
                 $state: &$lt_state T,
-                $iter: TokenIter<$lt_source>
+                $iter: &mut TokenIter<$lt_source>
             ) -> Option<Self> {
                 let mut $iter = $iter.clone();
                 $body;