[stdlib] Used map and guard for nil handling

[codestergit]

Changed nil handling by using map statements.
I think these changes should have guard let or map as it is more expressive.
Please let me know if anything else needed.
Thanks to swift team

[NachoSoto]

This could be implemented using Optional.map:

public mutating func next() -> Element? {
  return base.next().map { (index: count++, element: $0) }
}

[gribozavr]

-1 from me on this one because of side effects inside map().

[codestergit]

@gribozavr Changed it to guard let. What side effect it cause?

[gribozavr]

The increment in index++ was side effect.

[codestergit]

@NachoSoto Thinking about that, also find these changes in #193. I am making these changes. Thanks for suggestion.

[codestergit]

Done changes to handle nil by map.

[codestergit]

@gribozavr Thanks for reviewing this. I have done the changes. I have also done some performance testing and I get same results. All if/nil , map and guard are tanking almost same amount of time. You can find gist here
Please let me know if any other changes required.
I have run the test on apple swift master branch(this does not include my changes) and found some of test are failing on my machine. Here is output. Build is fine but do you know what stupid thing I have done while running the tests. I run same test with my changes and I got same response.
Thanks for your help

[gribozavr]

@codestergit I ran the tests and couldn't reproduce the failure.

[codestergit]

@gribozavr Thank for merging. Sorry but just for my knowledge want to know the index++ side effect. I am not that much good with higher order functions.How map will affect or behave different way. Optional.map has following implementation

public func map<U>(@noescape f: (Wrapped) throws -> U) rethrows -> U? {
    switch self {
    case .Some(let y):
      return .Some(try f(y))
    case .None:
      return .None //I think it will return as it is and f will not execute and `index++` should not happen
    }
  }

so it is also returning early if our base.next() return nil value or .None. It will not execute clousre and count value should remain same. I have tested this and both are giving same result. Is I missing something. Thanks for being so kind :)

[gribozavr]

@codestergit It should behave correctly just as you expect, it is just poor style to use side-effecting operations with APIs influenced by the functional paradigm, like map().

[codestergit]

Thanks @gribozavr .