Improve effe.Decision to accept step directly

[maratori]

Let's make it possible to write following flow

func A() error {
	effe.BuildFlow(
		effe.Decision(step1, effe.Failure(failure),
			effe.Case("a", effe.Step(step2)),
			effe.Case("", effe.Step(step3)),
		),
	)
	return nil
}

instead of current

effe/testdata/Decision/foo/effe.go

Lines 7 to 16 in d8f24ab

	func A() error {
	effe.BuildFlow(
	effe.Step(step1),
	effe.Decision(new(a), effe.Failure(failure),
	effe.Case("a", effe.Step(step2)),
	effe.Case("", effe.Step(step3)),
	),
	)
	return nil
	}

The difference is that Decision applies function with single return value as the first argument.