gmio_core: add gmio_stringstream() constructor

src/gmio_core/internal/stringstream.c

@@ -17,6 +17,16 @@
 
 #include "helper_stream.h"
 
+struct gmio_stringstream gmio_stringstream(
+        const struct gmio_stream stream, const struct gmio_string strbuff)
+{
+    struct gmio_stringstream sstream = {0};
+    sstream.stream = stream;
+    sstream.strbuff = strbuff;
+    gmio_stringstream_init(&sstream);
+    return sstream;
+}
+
 void gmio_stringstream_init(struct gmio_stringstream *sstream)
 {
     /* Trick: declaring the buffer exhausted will actually trigger the first

src/gmio_core/internal/stringstream.h

@@ -38,7 +38,15 @@ struct gmio_stringstream
             void* cookie, const struct gmio_string* strbuff);
 };
 
-/*! Initializes iterator */
+/*! Returns an initialized gmio_stringstream object */
+struct gmio_stringstream gmio_stringstream(
+        const struct gmio_stream stream,
+        const struct gmio_string strbuff);
+
+/*! Initializes position indicator
+ *
+ *  TODO: rename to gmio_stringstream_initpos()
+ */
 void gmio_stringstream_init(struct gmio_stringstream* sstream);
 
 /*! Returns the char where the iterator is currently pointing at */