Add map argument to MagickWand::import_image_pixels

[jshrake]

... and update the pixels argument to a slice.

Now, callers can specify the pixel layout and call this function without an owned Vec.

[nlfiedler]

Thanks for the fix. I'll add a note to the changelog that this is a breaking change. Not a concern, I just want to make sure it's made known. This crate is pre-1.0 so these sorts of changes are to be expected.