Have the State type be converted to String for storage in History API

[hgzimmerman]

Description

The state type is usually (), because support for actually storing data in the history api is difficult due to only JS types found in StdWeb being supported.

Instead, always store a string, and use Serde to transform your data into and out of JSON (or possibly bincode - maybe a feature flag for this?)

This shouldn't change the external interface too much, except that now types other than () and String would be able to be possible to store. I think the bound on JsSerializable should be dropped.