effectful 0.2.0

Effect<A, E, R> (sync + async), context/layers, pipe — interpreter-style, no bundled executor
Documentation 
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//! Ex 028 — Failed layers propagate `Err` from `build`.
use effectful::{LayerBuild, LayerFn, Tagged, Service};

#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash, Service)]
struct K;

fn main() {
  let bad = LayerFn(|| Err::<Tagged<K, u8>, &'static str>("no"));
  assert_eq!(bad.build(), Err("no"));
  println!("028_layer_build_errors ok");
}