FR: Calculate digest and diffid outside of oci_image

[thesayyn]

Currently, when oci_image assembles an image, it spends most of the time calculating DiffID and Digest of a layer. Since everything is being done in a single action, calculations are repeated everytime an input changes.

This is not ergonomic in terms of cacheability of these calculations and parallelization of machine resources.

[thesayyn]

fixed by #558