gstreamer: make gst_value_XXX() functions mapping more Ruby-ish

We map gst_value_XXX() functions to Gst::ValueFunctions.#XXX. And we
wrap them as methods like:

   module Gst::ValueMethods
     def XXX
       Gst::ValueFunctions.XXX(self)
     end
   end

Then we extend Gst::ValueMthods for GValue returned by GStreamer. For
example, GValues returned from Gst::Struct#fields.