Draft: Zap coercions in the rewriter

Sad about stalling in !6476, I have made my own attempt, after a short brainstorming session with @simonpj and having learned from @sheaf's attempts.

This zaps coercions right in the rewriter. It does repeat a fair bit of code. There may be an opportunity for improvement. Right now, I'm curious about performance. Note that this initial version unconditionally uses zapped coercions in the rewriter; a future version would have users choose via a compilation switch.

Meant to fix #8095.