Feature/incremental producer

[andyborn]

First draft, a couple questions around meta mod times as TODOs. Ended up being more similar to the normal Producer than expected, so might be able to refactor in a follow up.

[dim]

K, I checked something and it works. There is no need to make ProducerState public.

1. rename to producerState
2. and embed as value not as pointer

type Producer struct {
  producerState
  // ...
}

[dim]

here's what I have done to confirm:

// main.go
package main

import "x/internal"

func main() {
	x := new(internal.X)
	x.Inc()
	x.Inc()
	println(x.N()) // => 2
}

// internal/types.go

package internal

type X struct {
	y
}

type y struct {
	n int
}

func (y *y) Inc() {
	y.n++
}

func (y *y) N() int {
	return y.n
}

[andyborn]

hmmm, interesting.