Prototype using explicit padding for cbuffers

[bogner]

There are a few issues open regarding the cbuffer access pass, and the complexity for both DirectX and SPIRV is higher with the dx.Layout than we were hoping for. We should spend a little time prototyping the "explicit padding" alternative solution described in #171 to see if we should change course here.

[bogner]

TLDR, we should do this. Filing issues for a proposal and the work itself.