[#310] *: Implement string converters for enumerations

Implement `String` / `FromString` method pair in all levels of enum
definitions. From now `String()` returns canonical protojson-compatible
values.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

v2/object/string.go

@@ -1,29 +1,55 @@
 package object
 
-const (
-	typeRegularString      = "Regular"
-	typeTombstoneString    = "Tombstone"
-	typeStorageGroupString = "StorageGroup"
+import (
+	object "github.com/nspcc-dev/neofs-api-go/v2/object/grpc"
 )
 
+// String returns string representation of Type.
 func (t Type) String() string {
-	switch t {
-	default:
-		return typeRegularString
-	case TypeTombstone:
-		return typeTombstoneString
-	case TypeStorageGroup:
-		return typeStorageGroupString
-	}
+	return TypeToGRPCField(t).String()
 }
 
-func TypeFromString(s string) Type {
-	switch s {
-	default:
-		return TypeRegular
-	case typeTombstoneString:
-		return TypeTombstone
-	case typeStorageGroupString:
-		return TypeStorageGroup
+// FromString parses Type from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (t *Type) FromString(s string) bool {
+	var g object.ObjectType
+
+	ok := g.FromString(s)
+
+	if ok {
+		*t = TypeFromGRPCField(g)
 	}
+
+	return ok
+}
+
+// TypeFromString converts string to Type.
+//
+// Deprecated: use FromString method.
+func TypeFromString(s string) (t Type) {
+	t.FromString(s)
+	return
+}
+
+// String returns string representation of MatchType.
+func (t MatchType) String() string {
+	return MatchTypeToGRPCField(t).String()
+}
+
+// FromString parses MatchType from a string representation.
+// It is a reverse action to String().
+//
+// Returns true if s was parsed successfully.
+func (t *MatchType) FromString(s string) bool {
+	var g object.MatchType
+
+	ok := g.FromString(s)
+
+	if ok {
+		*t = MatchTypeFromGRPCField(g)
+	}
+
+	return ok
 }