Implement parent() on syntax_pos::Span

... and reimplement proc_macro::Span::parent using it. This function turns out
to be useful in the compiler as well

src/libproc_macro/lib.rs

@@ -270,7 +270,7 @@ impl Span {
     /// `self` was generated from, if any.
     #[unstable(feature = "proc_macro", issue = "38356")]
     pub fn parent(&self) -> Option<Span> {
-        self.0.ctxt().outer().expn_info().map(|i| Span(i.call_site))
+        self.0.parent().map(|x| { Span(x) })
     }
 
     /// The span for the origin source code that `self` was generated from. If

src/libsyntax_pos/lib.rs

@@ -291,6 +291,12 @@ impl Span {
         self.ctxt().outer().expn_info().map(|info| info.call_site.source_callsite()).unwrap_or(self)
     }
 
+    /// The `Span for the tokens in the previous macro expansion from which `self` was generated,
+    /// if any
+    pub fn parent(self) -> Option<Span> {
+        self.ctxt().outer().expn_info().map(|i| i.call_site)
+    }
+
     /// Return the source callee.
     ///
     /// Returns None if the supplied span has no expansion trace,