is duplicated by:

• SR-6306 Return == and != for optionals

relates to:

• SR-1482 Compare arrays of optionals
• SR-1997 Tuples with optional parameter aren't equatable
• SR-2284 Cannot invoke 'XCTAssertEqual' with an argument list of type '([Array], [[Int]])'
• SR-3055 Switch statements fail to test optionals for equality

Issue Description:

There are overloads of == for optionals and for non-optional arrays, but there's no overload for comparing optional arrays. This seems like a small oversight but would be useful broadly.

For example, this will not compile:

let a: [Int] = [1,2]
let b: [Int]? = [2,3]
let c: [Int] = [1,2]
let q = (a == b) // Error: value not unwrapped...
let x = (a == c)

Providing an == overload for optional arrays would tie up the loose ends left between the existing overloads:

@warn_unused_result
public func ==<Element : Equatable>(lhs: [Element]?, rhs: [Element]?) -> Bool {    
    switch (lhs, rhs) {    
    case (.Some(let l), .Some(let r)):    
        return l == r    
    case (nil, nil):    
        return true    
    default:    
        return false    
    }    
}    
    
@warn_unused_result    
public func !=<Element : Equatable>(lhs: [Element]?, rhs: [Element]?) -> Bool {    
    return !(lhs == rhs)    
}

let a: [Int] = [1,2]
let b: [Int]? = [2,3]
let c: [Int] = [1,2]
let q = (a == b) // a is promoted to []?
let x = (a == c)

This change would be added to Arrays.swift.gyb, so it would need to be modified to conform to the .gyb syntax, and would thus also extend ArraySlice.