feat(function): tap utility

[innocenzi]

This PR adds a new utility function, tap. This is a common utility found in ecosystems like Laravel or Ruby. Underscore has an implementation as well.

The concept is fairly simple: you give a value as the first argument to tap, and a callback as the second argument. The callback takes the value as a parameter, and is executed just before tap returns the initial value.

This is useful for avoiding temporary variables:

// Without tap
function createUser(name: string, age: number) {
  const user = new User()
  user.name = name
  user.age = age

  return user
}

// With tap
function createUser(name: string, age: number) {
  return tap(new User, user => {
    user.name = name
    user.age = age
  })
}

This is the first example that came to my mind, and as you can see, the trade-off tap offers to avoid declaring a variable is a little bit of readability. Sometimes, it's better not to use it, but most of the times, I find it super useful.

[innocenzi]

I applied the suggestions 👍

[innocenzi]

Thanks Anthony!