[Traversable] Remove that concept entirely

That concept was added more for completeness than for actual usefulness,
since I have not used it a single time since its creation. Given the
complexity of the library, I think it is better to remove it and shrink
the library to a more useful core than giving too many features that
contribute to bloat.

Also, the laws were difficult to check and it hence Traversable models
were not very well tested anyway.

I can always roll this back if someone shows me how Traversable is
actually useful for metaprogramming.

example/optional.cpp

@@ -86,26 +86,6 @@ BOOST_HANA_CONSTANT_CHECK(empty<Optional>() == nothing);
 
 }{
 
-//! [traversable]
-BOOST_HANA_CONSTEXPR_LAMBDA auto replicate3 = [](auto x) {
-    return make<Tuple>(x, x, x);
-};
-
-BOOST_HANA_CONSTEXPR_CHECK(
-    traverse<Tuple>(just(1), replicate3)
-    ==
-    make<Tuple>(just(1), just(1), just(1))
-);
-
-BOOST_HANA_CONSTANT_CHECK(
-    traverse<Tuple>(nothing, replicate3)
-    ==
-    make<Tuple>(nothing)
-);
-//! [traversable]
-
-}{
-
 //! [foldable]
 BOOST_HANA_CONSTEXPR_LAMBDA auto plus = [](auto x, auto y) {
     return x + y;