Derive Macro Monoid 

#[derive(Monoid)]

Derive macro for Monoid.

Implements Monoid for a struct by constructing empty() from each field’s empty.

Example

#[derive(Semigroup, Monoid)]
struct Foo {
    a: i32,  // i32: Monoid (empty = 0)
    b: Vec<String>,  // Vec: Monoid (empty = [])
}