Change $$iterator and $$asyncIterator to unique symbols in declaration file

[stephentuso]

From the docs:

To enable treating symbols as unique literals a new type unique symbol is available

That seems appropriate here. Additionally, only unique symbols can be referenced in types, e.g.

import { $$asyncIterator } from 'iterall'

type IterallAsyncIterable<T> {
  [$$asyncIterator]: AsyncIterator<T>
}

[coveralls]

Coverage remained the same at 100.0% when pulling f3e0e3d on stephentuso:unique-symbols into 76ce9a5 on leebyron:master.