Remove some non-essential trait re-exports from the prelude.


* `Default` only contains a static method. It provides no extension methods that are commonly used in client code.
* The `from_str` function is already re-exported, so re-exporting `FromStr` is not as necessary.
* The `iter_bytes` method on `IterBytes` is not needed all that much in client code, only really when data structures themselves are being implemented.
* `ToStrRadix` is being phased out in favor of using `std::fmt::radix`, and the `to_str_radix` method is rather specialised to warrant a global re-export.