Macro pl_lens::lens

macro_rules! lens {
    #[proc_macro_hack] => { ... };
}

This is a macro-based shorthand that allows us to write:

  lens!(SomeStruct.foo.bar_vec[3].baz)

instead of:

  compose_lens!(SomeStructFooLens, FooBarVecLens, vec_lens::<BarThing>(3), BarThingBazLens)