Rename `Formatter<>` to make it log-specific

[JayBazuzi]

Don't pollute the namespace by using a general name for such a specific concept.

Rename to LogFormatter<> or nest it inside Log, so it becomes Log.Formatter<>.

[jonsequitur]

The use of a nested class as a namespace always seems a little strange to me.

The rename makes sense but it would be a breaking change. There's a list of such changes that I'd like to make for 3.0, depending on people's interest.