Export KeyList type

[b-tarczynski]

For performance reasons I want to write custom encoder that doesn't include golang gob. I propose to export keyList type. It would simplify custom encoder implementation for keyList type without use of reflect package.

[timshannon]

Yeah if you want to do something like hardcoded byte returns or something this would be necessary. Good call. You'll also likely have to implement your own Storer interface to avoid reflection as well.