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.

`FromStr` already has its only static method, `from_str`, re-exported in the prelude. Implementing the trait itself is less common.

`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.

r? @brson