rustc_serialize: merge collection_impls.rs into serialize.rs.

`serialize.rs` has the `Encodable`/`Decodable` impls for lots of basic
types, including `Vec`. `collection_impls` has it for lots of collection
types. The distinction isn't really meaningful, and it's simpler to have
them all in a single file.

Author: nnethercote

compiler/rustc_serialize/src/collection_impls.rs

@@ -21,7 +21,6 @@
 
 pub use self::serialize::{Decodable, Decoder, Encodable, Encoder};
 
-mod collection_impls;
 mod serialize;
 
 pub mod leb128;