Switch to using slog

[sethvargo]

@yolocs this is basically what it would look like to migrate from zap->slog. We drop 4 deps and gain 1, so net change is 3. Once slog moves to stdlib, we can drop the dependency on golang.org/x/exp.

It's largely just a find-and-replace for most things except zap.Errorw. slog.Error actually accepts an "error" whereas zap.Errorw was variadic.

[sethvargo]

Not gonna merge this into slog is in the stdlib

[sethvargo]

I'm gonna close this out until we're closer to 1.21 (around August)