Use std.color based images type of `Slice!(2, XXX*)`

[9il]

Benefits:

1. Faster iterations (the color dimension becames CT loop).
2. Explicit image type system is less buggy for devs and users
3. Single conversion shell based on std.color can be used instead of set of conversion.

std.color can be improved to support DCV if it is required. In addition, we can add fastmath to std.color if we want.