add doc: string tupele support, hash support

source/libs/string/basic_string/std-tuple_element.rst

@@ -0,0 +1,45 @@
+.. _sprout-string-basic_string-std-tuple_element:
+###############################################################################
+std::tuple_element
+###############################################################################
+
+Interface
+========================================
+.. sourcecode:: c++
+
+  namespace std {
+    template<std::size_t I, typename T, std::size_t N, typename Traits>
+    struct tuple_element<I, sprout::basic_string<T, N, Traits> >;
+  }
+  
+  // syntax
+  std::tuple_element<I, basic_string<T, N, Traits> >::type
+
+Requires
+========================================
+
+| ``I < N``. The program is ill-formed if I is out of bounds.
+
+Value
+========================================
+
+| The type T.
+
+Examples
+========================================
+.. sourcecode:: c++
+
+  #include <sprout/string.hpp>
+  #include <type_traits>
+  using namespace sprout;
+  
+  using type = string<8>;
+  using element_type = std::tuple_element<0, type>::type;
+  static_assert(std::is_same<element_type, char>::value, "tuple element type of string is char.");
+
+Header
+========================================
+
+| ``sprout/string/tuple.hpp``
+| Convenience header: ``sprout/string.hpp``
+