Move some/all array extensions to Collection/Sequence/Indexable protocols

[piv199]

.reserved (need to think what where to move)

forEachEnumerated(_:) -> Sequence
get(at:) -> BidirectionalCollection, can be Collection, but I don't think it's good enough, bidirectional is better
containsType(of:) -> Collection
insertFirst(_:) -> Array, This fits array mostly
random(_:) -> Collection, think of Dictionary return tuple
reverseIndex(_:) -> Collection, stick with where Self.Index == Int ?

[piv199]

Dict seems not to be good fitted for containsType(of:)
But Set fits great.

   let dict: [String: Int?] = ["a": 1, "b": 2, "c": nil]
   let hash = Set<String>.init(["a", "b", "c"])

   let res1 = dict.containsType(of: "b") //false
   let res11 = dict.containsType(of: 2) //false
   let res123 = dict.containsType(of: ("a", 3)) //false
   let res1234 = dict.containsType(of: (String, Int?)("a", 3)) //true
   let res2 = hash.containsType(of: "c") //true

[piv199]

    public func random() -> Iterator.Element? {
// add guard for empty collections
        let rnd = Int(arc4random_uniform(UInt32(underestimatedCount)))
        return self[index(startIndex, offsetBy: count.advanced(by: -1 - rnd))]
    }