schema: Add NewLabel function.
schema: Make Labels sortable by name.
schema: Add a Labels Set function.
discovery: convert App.Labels from map to types.Labels.

As App will be used by various functions in the future, make it consistent with
the spec and use types.Label.
In this ways the labels can be validated.
discovery: Add a NewStringFromApp function.

This functions will be used in future.
The name will be reproducible: apps with same name and labels will have the
same string.
The first labels will be (in order): version, os, arch and the other labels
will be ordered by name.
Revert "spec: default digest format is sha512"

This reverts commit b7743540b9a670870281ae5cc6cabaaf5c822b31.

Conflicts:
	SPEC.md
	examples/image.json