add nonempty encoding for spans

Before this change, encoding an object containing a codemap::span
using the JSON encodeng produced invalid JSON, for instance:
[{"span":,"global":false,"idents":["abc"]}]
Since the decoder for codemap::span's ignores its argument, I
conjecture that this will not damage decoding, and should improve
it for many decoders.

src/libsyntax/codemap.rs

@@ -140,7 +140,7 @@ impl cmp::Eq for span {
 
 impl<S:Encoder> Encodable<S> for span {
     /* Note #1972 -- spans are encoded but not decoded */
-    fn encode(&self, _s: &S) { }
+    fn encode(&self, _s: &S) { _s.emit_nil() }
 }
 
 impl<D:Decoder> Decodable<D> for span {