Proposal for explicit padding type and its use in cbuffers

[bogner]

Write a proposal for the explicit padding approach to cbuffers and HLSL-specific struct offsets. This will supercede #171 (as one of the alternative solutions there) and remove the need for dx.Layout and its SPIR-V equivalent.

What needs to be covered:

• The explicit padding type and alternatives on how to spell it (A first class type, a specially named struct, or a target type)
• How this will be used for packoffset in cbuffers
• How this will be used for explicit cbuffer-style arrays
• Implications on what reordering should be allowed by things like packoffset
• Usefulness for vk::offset