NXT design for memory barriers and buffer mapping.

Hey all,

We wrote some document to help everyone reason about NXT's proposals for
memory barriers and resource upload /download. Unfortunately we still don't
have a fleshed out proposal that minimizes the number of copies on UMA.
Instead the docs focus on explaining our current design for resource
upload/download and for memory barriers since they are very tied.
Eventually we'll have these docs in MarkDown in some repo, either WebGPU's
or NXT's.

   - NXT "memory barriers"
   <https://docs.google.com/document/d/1k7lPmxP7M7MMQR4g210lNC5TPwmXCMLgKOQWNiuJxzA>
   <- Please read this first as buffer mapping depends on it.
   - NXT buffer mapping
   <https://docs.google.com/document/d/1HFzMMvDGHFtTgjNT0j-0SQ1fNU9R7woZ4JuNJdAXBjg>

Cheers,

Corentin