RFC: proposal for declarative VDUs

[jcoglan]

This PR contains the design documents we have produced as part of the M4 milestone for the STA project to design a declarative way to express document update validation functions. This was originally written as an "exploratory" or "rationale" document to compare different approaches and discover features that would be necessary to adequately cover the needed functionality, and then a shorter "specification" describing just the features we're proposing to build. At @janl's suggestion these documents form the "Advantages and Disadvantages" and "Detailed Description" parts of the RFC respectively.