In swift 1.2, `Slice` has been renamed `ArraySlice`.
In swift 2.0, type parameter name of `Array` is changed from `T` to `Element`.
Therefore, modified the names which appear in code comments and a document.
This draft needs to be reviewed by Chris and Joe, who both did much of
the design work but haven't seen the text. In particular, the
"Optimization" section is a bit hand-wavey and might benefit from being
beefed up by a core compiler engineer.
Swift SVN r13730
