TryFrom Parts for Uri has a Error type that can never occur

[seanmonstar]

The current implementation has type Error = InvalidUri, but this can never occur.

If we decide that #96 should be changed or removed, then this could be updated to correctly return an InvalidUri error.

If not, we might want to consider that a From<T> will likely end up in a TryFrom<T> where Error = ! blanket impl. Can we have some uninhabited type in convert.rs as the Error, and then alias it to bang or something later?

[carllerche]

Handled by #114

[carllerche]

Fixed by #114