add `join` operator to `IPure<T>`

[fogfish]

The lib supports stack construction using following syntax

const app = new cdk.App()
const stack = (): cdk.StackProps => ({})
pure.iaac(cdk.Stack)(stack)
  .effect(x => {
    pure.join(x, RestApi)
    pure.join(x, LogGroup)
    pure.join(x, CodeBuildEvents)
  })
app.synth()

The code become cleaner if join is part of IPure

pure.iaac(cdk.Stack)(stack)
  .join(RestApi)
  .join(LogGroup)
  .join(CodeBuildEvents)