Added some more cases to the test, and implemented a new lint,
cast_possible_wrap, triggered when casting from an unsigned type to a
signed type of the same size.
Extracted the match that determines an integer types's size in a
utility function and implemented support for usize/isize.
Added a needed feature to the crate root.
Added some tests to cover those cases, and a test I previously forgot.
Silenced two errors signaled by dogfood.sh in unicode.rs.
Renamed the cast_possible_overflow lint to cast_possible_truncation,
and updated the error message, readme and crate root accordingly.
Added some more information to the message for the cast_precision_loss
lint.
Updated the test case to reflect changes.
