feat(equals): derive anf optimized "equals function" from your @traverable schemas

[ahrjarrett]

With this PR, users can import Eq from @traversable/derive-equals and use Eq.fromSchema to get an "equals function" for free.

const Todo = t.object({
  complete: t.boolean,
  title: t.string,
  body: t.optional(t.array(t.string)),
})
interface Todo extends t.typeof<typeof Todo> {}
const todoEquals = Eq.fromSchema(Todo) 
//     ^? const todoEquals: Eq<{ complete: boolean, title: string, body?: string[] }>
const A: Todo = {
  complete: false,
  title: 'Todo A',
  body: [
    'Line 1',
    'Line 2',
    'Line 3',
  ]
}
const B: Todo = {
  ...A,
  body: [
    ...A.body,
  ]
}
const C: Todo = {
  ...A,
  body: [
    ...A.body,
    'Line 4',
  ]
}

// success:
console.log(todoEquals(A, B)) // => true
console.log(todoEquals(A, A)) // => true
console.log(todoEquals(B, B)) // => true

// failure:
console.log(todoEquals(A, C)) // => false
console.log(todoEquals(B, A)) // => false