extra::ebml: Make reader::Res public

Since reader::vuint_at() returns a result of type reader::Res it makes sense
to make it public.

Due to rust's current behavior of externally referenced private structures,
https://github.com/mozilla/rust/issues/10573, you could still use the result and
assign it to a variable if you let the compiler do the type assignment,
but you could not explicitly annotate a variable to hold a reader::Res.

src/libextra/ebml.rs

@@ -90,7 +90,7 @@ pub mod reader {
 
     // ebml reading
 
-    struct Res {
+    pub struct Res {
         val: uint,
         next: uint
     }