Replace aggregation types with functions

[semistrict]

Fixes: #399

[odeke-em]

I think only Distribution should be a function, the rest can be constants e.g https://play.golang.org/p/ScwUIbF2Tm4
or inlined

package main

type Aggregation interface {
	aggregate()
}

type countAggregation int

func (countAggregation) aggregate() {}

const CountAggregation = countAggregation(0)

func main() {
	var _ Aggregation = CountAggregation
}

[semistrict]

discussed with @rakyll yesterday and she preferred to have all of them be functions for consistency.

[odeke-em]

Thank you both for the discussion and for the discourse. I see my idea is ugly for consistency as well as for the godoc itself. See

[odeke-em]

Thank you @Ramonza and @rakyll for the discourse, I am now convinced that functions are the best option for consistency and a great abstraction to avoid leaking the underlying implementation as displayed by the docs. LGTM @Ramonza.

I've added the labels for api-breaking and do not merge just to keep us aware.

[semistrict]

verified that this does not break cloud.google.com/go