[Searchable] Refactor the interface

- Rename elem to contains
- Rename subset to is_subset, and make is_subset applicable in infix notation
- Add the at_key method
- operator[] is now bound to at_key instead of find

example/tutorial/include_set.cpp

@@ -13,7 +13,7 @@ using namespace boost::hana;
 
 
 int main() {
-    BOOST_HANA_CONSTEXPR_LAMBDA auto xs = set(1, 2.2, 'x');
-    BOOST_HANA_CONSTEXPR_CHECK(elem(xs, 'x'));
+    constexpr auto xs = set(1, 2.2, 'x');
+    static_assert(contains(xs, 'x'), "");
 }
 //! [main]