add spec and workflow diff utils

[essweine]

This adds some utilities for

• comparing workflow specs
• identifying tasks in a workflow instance affected by changes to the spec

The spec comparison utility uses a serializer to compare two versions of a spec; the serializer already knows about all the attributes of a particular ask spec so it can be leveraged to avoid implementing attribute comparisons for each type of spec.

The comparisons provides lists of

• specs added in the later version
• specs removed in the later version (or tasks that use removed specs)
• specs that changed between versions (or tasks that use the changed specs)
• an alignment mapping specs from the original to the later version (or from a task to the later version of a task's spec)

This is intended to provide information about whether a spec can be updated on an existing workflow.