ExpressibleByArrayLiteral and ExpressibleByDictionaryLiteral conformances

[fwcd]

It would be very convenient to have these conformances, since non-empty collections could be initialized as naturally as the built-in collection types:

let x: NonEmpty<[Int]> = [1, 2, 3]
let y: NonEmpty<[Int: String]> = [1: "abc", 2: "def"]

However, since ExpressibleByArrayLiteral and ExpressibleByDictionaryLiteral don't allow their initializers to fail, we would have to crash at runtime, which might not be desirable given the description of the library as a compile-time guarantee:

init(arrayLiteral: Element...) {
  guard !arrayLiteral.isEmpty else { fatalError("Non-empty array literal!") }
  self.init(rawValue: arrayLiteral)!
}

or

init(arrayLiteral: Element...) {
  assert(!arrayLiteral.isEmpty, "Non-empty array literal!")
  self.init(rawValue: arrayLiteral)!
}

Thoughts?

[stephencelis]

@fwcd This sounds totally reasonable to us! Want to draft a PR implementing the functionality?

[fwcd]

Sure!