Rethink the dlog.Logger interface

[LukeShu]

The 1.x dlog.Logger interface is simple-to-implement but "slow" in that non-logged formatting can't be optimized out. Implementations can opt-in to such an optimization by also implementing the dlog.OptmizedLogger interface, but it's more complex.

There's some thinking that it's a design flaw that it's so easy to do the slow thing.

See the discussion on #33

[LukeShu]

Related art: logr.LogSink https://pkg.go.dev/github.com/go-logr/logr#LogSink

logr.LogSink has an .Enabled(level) method that can be used to implement this optimization, without delegating formatting to the backend. However, this limits the backend to only be able to filter based on the loglevel, and nothing else.