Results 2 issues of Kamil

While using rodio with symphonia features enabled values reported by [Decoder::total_duration](https://docs.rs/rodio/latest/rodio/decoder/struct.Decoder.html#method.total_duration) are incorrect. It works fine while using other backends. Calculating it with symphonia directly also return correct value. Minimum...

bug

This PR implements checked arithmetic operations for newtypes whose inner types are integers. It adds an attribute, similar to `new_unchecked` that enable this. I considered implementing this as a `num_traits`...