Macro assign_impl

macro_rules! assign_impl {
    ($a:ident += $b:ident) => { ... };
    ($a:ident -= $b:ident) => { ... };
    ($a:ident *= $b:ident) => { ... };
    ($a:ident /= $b:ident) => { ... };
}

Implement an assign trait based on an existing non-assign trait.