Compare and contrast with old proposals

[js-choi]

@acutmore pointed out in Matrix a few days ago that the explainer does not address the differences this proposal has with the old Stage-0 bind proposal.

In fact, this should be the top thing that the explainer does: it’s a simplified resurrection of the old bind proposal, and it should compare and contrast itself with the old bind proposal (and with the Stage-1 Extensions proposal; see #9).

This was stubbed out in fc74301. Still need to finish.