Macro easy_conv::chained_into

macro_rules! chained_into {
    ($source: ty, $($tail: ty),+) => { ... };
    (@impl $source: ty | $($intermediate: ty),* | $target: ty) => { ... };
    (@get_target $target: ty) => { ... };
    (@get_target $_: ty, $($tail: ty),+) => { ... };
    (@next $val: expr, $intermediate: ty) => { ... };
    (@next $val: expr, $intermediate: ty, $($tail: ty),+) => { ... };
}

Implement conversion from A to Z via B, C, …

B needs to implement From<A>, C needs to implement From<B>, and so on.

Example: chained_into!(A, B, C, D, E); implements From<A> for E by converting A to B, then B to C, then C to D, and finally D to E.