"Taking control of SFINAE" does not showcase Optional's strengths properly

It was pointed on the [Boost.Devel mailing list](http://article.gmane.org/gmane.comp.lib.boost.devel/261160) that the "Taking control of SFINAE" section of the tutorial could have been achieved more simply with `overload_linearly`:

``` c++
auto optionalToString = hana::overload_linearly(
[](auto&& x) -> decltype(x.toString()) { return x.toString(); },
[](auto&&) -> std::string { return "toString not defined"; }
);
```

My initial goal was to showcase `Optional`'s strengths, but clearly that example is not the right one.

Start by reading the tutorial's "Taking control of SFINAE" section and the linked Boost.Devel discussion. The issue names no file or replacement example; done means revising that section so it showcases Optional's strengths more appropriately.

Written by the indexing model from the issue text.