Add cxx{in,out}type typemaps and use them for std::string

This makes using returning strings much simpler to use from C++ code as
the returned pointers don't have to be deleted manually -- although, of
course, this does require an extra allocation and copy and so should be
avoided for the very long strings.

Add a new runtime test showing how simple and convenient it is to use
the functions working with string using the C++ wrappers now.

Lib/c/std_string.i

@@ -27,6 +27,10 @@ public:
 };
 }
 
+%fragment("include_string", "cxxheader") %{
+#include <string>
+%}
+
 namespace std {
 
 // use "const string &" typemaps for wrapping member strings
@@ -68,4 +72,19 @@ class string;
 // taking strings and raw pointers in the generated wrappers.
 %typemap(typecheck) string, const string &, string *, string & = char *;
 
+
+// Define typemaps for wrapping strings back into std::string in C++ wrappers
+// and accepting strings directly.
+
+%typemap(cxxintype, fragment="include_string") string, const string & "std::string const&"
+
+%typemap(cxxin) string, const string & "$1.c_str()"
+
+%typemap(cxxouttype, fragment="include_string") string, const string & "std::string"
+
+%typemap(cxxout, noblock="1") string, const string & %{
+  $result = std::string($cresult);
+  free(const_cast<char*>($cresult));
+%}
+
 }