pixelwand instead of (const pixelwand) in foreign function signatures

The const qualifier on a foreign type prevents that type from being
automatically boxed and unboxed.  We will need to make this same change
for a number of other types.
magickwand instead of (const magickwand) in foreign function signatures
drawingwand instead of (const drawingwand) in foreign function signatures
remove more incorrect uses of (const ...)