Add Memo, Mutable

[fredpi]

I suggest adding the Memo Type to HandySwift, based on this repo.

My suggested implementation adjusting the implementation a bit can be found here.

[Jeehut]

Could you please explain, how this would improve Swift with some usage examples and why you think "it should have been part of the Swift Standard Library"? Also, could you check if there were discussions about this (or something solving the same problem) at the new Swift Evolution Forum and link it here, if you can find it? Thank you in advance.

[fredpi]

As pointed out in #17, wrapper types aren't very swifty, in that they should be a part of the standard library. While Mutable would be needed for some rare use cases (as seen in the Memo implementation), its use shouldn't be encouraged by being a part of Swift itself.

Same goes for Memo, which, in fact, is just a lazy box somehow. So while useful, they both don't fit here. Closing...