Parse error types (like ParseIntError) don't seem to be available in the std namespace, just core

[sellibitze]

(judging by the API docs)

I'd like to be able to use Result<u8, ParseIntError> as a function's return type without having to use directly core::num::ParseIntError since this is what parse::<u8> apparently returns now and thus us a public facing type.

cc @alexcrichton

[alexcrichton]

Oops, sorry about that!