Macro dxpr::implement_grad

macro_rules! implement_grad {
    ($name:ty >-> $output:ty: |$self:ident, $x:ident| $body:expr) => { ... };
    ($($t:ident: $const_trait:path),+ => $name:ty >-> $output:ty: |$self:ident, $x:ident| $body:expr) => { ... };
    ($($t:ident: $const_trait:path),+ => $name:ty >-> $output:ty: |$self:ident, $x:ident| where own { $own:expr } else { $ref:expr }) => { ... };
}

Automagically implement Grad.