Have log.Fielder return a map[string]interface{} ?

Right now you can't implement a custom fielder without pulling in the `apex/log` library.

I'm not sure if this would be a breaking change or not, but it'd be nice to change:

```go
type Fielder interface {
Fields() Fields
}
```

to this:

```go
type Fielder interface {
Fields() map[string]interface{}
}
```