GMemoryOutputStream: Add new _resizable() constructor usable from bindings

Really, the memory output stream API is too warped around the model
where it's a fixed size buffer that you've already allocated.  Even in
C, I find myself always wanting to use it to just accumulate data into
an arbitrary-sized buffer it allocates.

Unfortunately, it's also not usable from bindings because it's not
common to bind g_free() and g_realloc(), but if you just pass NULL, you
get the default of a fixed size, which is useless as per above.

I am going to use this from a gjs test case, and the GSubprocess test
cases also will use it.

https://bugzilla.gnome.org/show_bug.cgi?id=688931

docs/reference/gio/gio-sections.txt

@@ -863,6 +863,7 @@ GBufferedOutputStreamPrivate
 GReallocFunc
 GMemoryOutputStream
 g_memory_output_stream_new
+g_memory_output_stream_new_resizable
 g_memory_output_stream_get_data
 g_memory_output_stream_get_size
 g_memory_output_stream_get_data_size