Add proposed binary format and initial prototype notes

[tlively]

With the intent to make the proposal concrete enough to implement.

[tlively]

cc @jakobkummerow.

I'm going to go ahead and merge this, but @rossberg, I would appreciate if you could take a look and provide any feedback on the proposed factoring of the syntax and binary format. Specifically, I add descriptor and describes clauses as prefixes of comptype just like we do with shared in the shared-everything threads proposal. Does that sound right to you?

[rossberg]

This mostly looks good to me. The only part that doesn’t seem right is the placement of descriptor clauses outside the shared clause. Sharedness “scopes over” the descriptor clauses, i.e., in a shared type, the descriptor and describee must be shared as well. I think the syntactic structure needs to reflect that.

Another bikeshedding comment is that I would replace the _rtt suffix in the new instructions with _desc. First, because that matches the terminology of the declaration clauses. And second, because descriptors are not actually RTTs, they merely carry them, plus arbitrary other stuff.

[tlively]

Thanks, @rossberg! These adjustments (and fixing the text format in the examples) is done in #3 and #4.