Macro impl_widen 

macro_rules! impl_widen {
    ($($from:ty => $to:ty),+ $(,)?) => { ... };
}

Implement infallible widening Coerce<$to> for each $from => $to pair via Into.

For widenings that have no From impl (platform-dependent isize/usize, lossy int→float), write the impl Coerce by hand with an as cast instead.