Macro layer_node 

macro_rules! layer_node {
    ($id:expr, requires = [$($req:expr),* $(,)?], provides = [$($prov:expr),* $(,)?]) => { ... };
}

Build a single LayerNode.

let node = effectful::layer_node!(
  "repo",
  requires = ["Db", "Cache"],
  provides = ["Repo"]
);