Prepared handlers

[foxfriends]

Rather than defining a regular function accepting a closure and then putting a handler inside it, dedicated syntax for preparing handlers around a deferred execution would be nice. It is just syntax sugar in the end, but needs two things:

• a way to define such a prepared handler
• a way to apply the prepared handler to the rest of the current sequence

Defining a prepared handler (no syntax yet, just use a regular proc-accepting-proc):

proc noop_logger!(body) {
  return with body!()
    when 'log(x) resume unit
    else yield
}

Applying a prepared handler:

proc no_log!() {
  use noop_logger
  yield 'log("hello")
}

Expands to:

proc no_log!() {
  noop_logger!(do() {
    yield 'log("hello")
  })
}