Improve error messages to show type names instead of numbers

Error messages for type mismatches now display readable type names like
'Map' and 'Slice' instead of numeric codes, making debugging easier.

Author: oschwald

internal/decoder/reflection.go

@@ -103,8 +103,7 @@ PATH:
 		case string:
 			// We are expecting a map
 			if typeNum != KindMap {
-				// XXX - use type names in errors.
-				return fmt.Errorf("expected a map for %s but found %d", v, typeNum)
+				return fmt.Errorf("expected a map for %s but found %s", v, typeNum.String())
 			}
 			for range size {
 				var key []byte
@@ -125,8 +124,7 @@ PATH:
 		case int:
 			// We are expecting an array
 			if typeNum != KindSlice {
-				// XXX - use type names in errors.
-				return fmt.Errorf("expected a slice for %d but found %d", v, typeNum)
+				return fmt.Errorf("expected a slice for %d but found %s", v, typeNum.String())
 			}
 			var i uint
 			if v < 0 {