[#55] refs: Add Scheme field to Signature

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

refs/string.go

@@ -24,3 +24,24 @@ func (t *ChecksumType) FromString(s string) bool {
 
 	return ok
 }
+
+// String returns string representation of SignatureScheme.
+func (t SignatureScheme) String() string {
+	return refs.SignatureScheme(t).String()
+}
+
+// FromString parses SignatureScheme from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (t *SignatureScheme) FromString(s string) bool {
+	var g refs.SignatureScheme
+
+	ok := g.FromString(s)
+
+	if ok {
+		*t = SignatureScheme(g)
+	}
+
+	return ok
+}