v2 suggestions

[peteski22]

• Lots of Go Doc tweaks
• Renamed GetLevel to Level to be more idiomatic Go (https://go.dev/doc/effective_go#Getters)
• Some receivers weren't using pointers (but others for the same type were), so I aligned them all to use pointers
• Tweaked the early return for log methods so they return if someone tries to log at Off level too
• I think there were bugs in values.go receivers for IsX checking for Debug all the time https://github.com/hashicorp/go-hclog/pull/143/files#diff-e7a71de986f0a776ec8024ba579fe753d2410f900cf9ecb9d05e37c3276e8c46R68-R95
• Some tests don't pass, but didn't seem to before either.

Just as a personal view, I'm not sure about the LogImpl name, but I don't know what's better, I added a // TODO: comment in there somewhere but it can be ignored in your PR if you'd like. I won't be too offended 😄