rwops: Remove Collection impl

The `Collection` trait was removed as part of the
collections reform in Rust master.

Instead, we impl it for `RWops` directly.

Author: crumblingstatue

src/sdl2/rwops.rs

@@ -72,6 +72,12 @@ impl RWops {
         if raw.is_null() { Err(get_error()) }
         else { Ok(RWops{raw: raw, close_on_drop: false}) }
     }
+
+    pub fn len(&self) -> uint {
+        unsafe {
+            ((*self.raw).size)(self.raw) as uint
+        }
+    }
 }
 
 impl Drop for RWops {
@@ -148,11 +154,3 @@ impl Seek for RWops {
         }
     }
 }
-
-impl Collection for RWops {
-    fn len(&self) -> uint {
-        unsafe {
-            ((*self.raw).size)(self.raw) as uint
-        }
-    }
-}