Make TypeAdapter.typeId a string

It looks like there may be a concern of type adapter collision if more than one adapter uses the same typeId.

If this is true, I'm wondering if it would be better if it was a String instead.

An easy migration path is to use the hashCode of `String typeId` to get an int value that could be used otherwise, then recommend that people give each type adapter a unique name.

If my understandings here are wrong, please let me know if this is documented somewhere.