Merge pull request #214 from wlynch/v0.2-type-alias

Add type aliases to v0.2 provenance to revert backwards incompatible change.

in_toto/slsa_provenance/v0.2/provenance.go

@@ -11,6 +11,13 @@ const (
 	PredicateSLSAProvenance = "https://slsa.dev/provenance/v0.2"
 )
 
+// These are type aliases to common to avoid backwards incompatible changes.
+type (
+	DigestSet          = common.DigestSet
+	ProvenanceBuilder  = common.ProvenanceBuilder
+	ProvenanceMaterial = common.ProvenanceMaterial
+)
+
 // ProvenancePredicate is the provenance predicate definition.
 type ProvenancePredicate struct {
 	// Builder identifies the entity that executed the invocation, which is trusted to have