Port from config-rs to figment (v2)

This is a new version of !1989 (closed), rebased onto !2040 (merged). This way, we can take !2040 (merged) now, and decide about the rest of our figment issues at our leisure.

Please see discussions at !1989 (closed) for open issues; this does not need a fresh review unless we decide to fix them and move ahead.

Closes #1267 (closed).

Closes #1268 (closed).