simplify repeated types with generics

[egonelbre]

Currently we create quite a lot of boilerplate that looks like:

	c.Message(1, func(c *picobuf.Decoder) {
		if m.Value == nil {
			m.Value = new(Message)
		}
		m.Value.Decode(c)
	})
	c.Message(2, func(c *picobuf.Decoder) {
		if m.Opt == nil {
			m.Opt = new(Message)
		}
		m.Opt.Decode(c)
	})
	c.RepeatedMessage(3, func(c *picobuf.Decoder) {
		x := new(Message)
		c.Loop(x.Decode)
		m.Rep = append(m.Rep, x)
	})
	c.PresentMessage(4, m.PresentBasic.Decode)
	c.PresentMessage(5, m.PresentOpt.Decode)
	c.RepeatedMessage(6, func(c *picobuf.Decoder) {
		m.PresentRep = append(m.PresentRep, Message{})
		c.Loop(m.PresentRep[len(m.PresentRep)-1].Decode)
	})

We should be able to convert these to something along the lines of:

	picowire.DecodeMessage[Message](c, 1, &m.Value)
	picowire.DecodeMessage[Message](c, 2, &m.Opt)
	picowire.DecodeRepeatedMessage[Message](c, 3, &m.Rep)
	c.PresentMessage(4, m.PresentBasic.Decode)
	c.PresentMessage(5, m.PresentOpt.Decode)
	picowire.DecodeRepeatedPresentMessage[Message](c, 6, &m.PresentRep)