syntax: add the OwnedSlice vector wrapper.

This is a stand-in until we have a saner `~[T]` type (i.e. a proper
owned slice). It's a library version of what `~[T]` will be, i.e. an
owned pointer and a length.

src/libsyntax/lib.rs

@@ -49,6 +49,7 @@ pub mod syntax {
     pub use parse;
 }
 
+pub mod owned_slice;
 pub mod opt_vec;
 pub mod attr;
 pub mod diagnostic;