libgraphviz: add label and escaped ctors taking any str::IntoMaybeOwned.

src/libgraphviz/lib.rs

@@ -421,6 +421,14 @@ fn edge_label(&'a self, e: &E) -> LabelText<'a> {
 }
 
 impl<'a> LabelText<'a> {
+    pub fn label<S:IntoCow<'a, String, str>>(s: S) -> LabelText<'a> {
+        LabelStr(s.into_cow())
+    }
+
+    pub fn escaped<S:IntoCow<'a, String, str>>(s: S) -> LabelText<'a> {
+        EscStr(s.into_cow())
+    }
+
     fn escape_char<F>(c: char, mut f: F) where F: FnMut(char) {
         match c {
             // not escaping \\, since Graphviz escString needs to