* This PR is against a branch called `main`
* Internally all docs/CI/etc is updated
* The default branch of the repo is now `main`
* All active PRs have been updated to retarget `main`
Closes#1914
Update the documentation for the merger, and also for various changes in
Cranelift. Remove some old obsolete documentation, and convert the remaining
Sphinx files to Markdown. Some of the remaining content is still out of
date, but this is a step forward.
The failure crate invents its own traits that don't use
std::error::Error (because failure predates certain features added to
Error); this prevents using ? on an error from failure in a function
using Error. The thiserror crate integrates with the standard Error
trait instead.
