Very faster version of rgb-to-gray

* Use of primitive function aget. set-pixel! seems to be faster than aset-int.
Faster image file loading

* The macro mult-aset is the faster function to set the pixels.
Minor changes
Faster to-buffered-image; function to get the raster of a BufferedImage
Improvements in the performance, use of mult-aset/mult-aget macros
Minor changes