Macro impl_per 

macro_rules! impl_per {
    ($($t:ident ($str:literal) per {$(
        $larger:ident : [$default_output:ty]

        $($int_output:ty)|+ = $int_value:expr;
        $($float_output:ty)|+ = $float_value:expr;
    )+})*) => { ... };
}

Declare and implement Per for all relevant types. Identity implementations are automatic.