Cannot cleanup exit handler inside the handler itself

[ehmicky]

Bug

The following:

import { onExit } from 'signal-exit'

const cleanup = onExit(() => {
  console.log('A')
  cleanup()
})

onExit(() => {
  console.log('B')
})

Prints:

A

But should print:

A
B

Reason

When the exit is happening, signal-exit is iterating over the exit handlers here:

signal-exit/src/index.ts

Line 107 in d8621bc

for (const fn of this.listeners[ev]) {

However, cleanup() ends up calling:

signal-exit/src/index.ts

Line 93 in d8621bc

list.splice(i, 1)

Which directly mutates this.listeners. This makes the iteration above not work anymore, due to mutating an array while iterating on it. Instead, the following would work:

    for (const fn of [...this.listeners[ev]]) {

[ehmicky]

PR at #86.