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.

Examples/test-suite/c/li_boost_shared_ptr_runme.cxx

@@ -5,11 +5,11 @@
 int main(int argc, const char *argv[]) {
     {
         li_boost_shared_ptr::Klass k("me oh my");
-        assert( strcmp(k.getValue(), "me oh my") == 0 );
+        assert( k.getValue() == "me oh my" );
     }
 
     {
         li_boost_shared_ptr::Klass k{li_boost_shared_ptr_factorycreate()};
-        assert( strcmp(k.getValue(), "factorycreate") == 0 );
+        assert( k.getValue() == "factorycreate" );
     }
 }

Examples/test-suite/c/li_std_string_runme.cxx

@@ -0,0 +1,26 @@
+#include "li_std_string_wrap.h"
+
+#include <assert.h>
+
+using namespace li_std_string;
+
+int main(int argc, const char *argv[]) {
+    Structure st;
+    assert( st.MemberString().empty() );
+
+    st.MemberString("bloordyblop");
+    assert( st.MemberString() == "bloordyblop" );
+
+    assert( st.MemberString2() == "member string 2" );
+
+    assert( st.ConstMemberString() == "const member string" );
+
+    st.StaticMemberString(std::string("static bloordyblop"));
+    assert( st.StaticMemberString() == "static bloordyblop" );
+
+    assert( Structure::StaticMemberString2() == "static member string 2" );
+    assert( Structure::ConstStaticMemberString() == "const static member string" );
+
+    Foo f;
+    assert( f.test("1+") == "1+1" );
+}