Make validators consistent

[davidyuk]

@sadiqevani Arrow functions should be enough obvious because they were introduced in ES6 four years ago. object-shorthand rule of Airbnb configs (that you have chosen in #160) rejects code like:

const t = {
  validator: function() {
    // ...
  }
}

at least should be used:

const t = {
  validator() {
    // ...
  }
}

but arrow functions, in this case, is better because it is shorter:

• no need to write return
• curly brackets are not necessary
• simple validators can be written in a single line

Also, functions starting with () => without curly brackets are easier to read, because:

• it has only one statement
• it can't define internal variables
• it can't have own this