DR and SR duplication and merging

[Skepfyr]

Currently, SR is built on top of DR, as DR is already written and it provides a useful and productive base to work on. However, SR may be simpler as a parser from binary blob form, so if SR is requiring large scale changes in DR code then it is worthwhile considering merging the two at that point to reduce duplicated effort.

[kvark]

I think DR has the following values today:

1. it's good for bootstrapping SR
2. it's fairly straightforward and sound in what it tries to do

However, when SR is there I don't think we'll see anybody wanting to use DR specifically as opposed to either raw spirv or SR. So in the long run we should probably just have just SR.