add import / export functions

[erdo]

The serialization code can be taken from https://github.com/erdo/persista

the key lines we'd need are:

val jsonText = json.encodeToString(serializer(stateKType), item)

json.decodeFromString(serializer(stateKType), jsonText) as NavigationState<L, T>

[erdo]

basically make these two (currently ignored) tests in NavigationExtensionsTest pass:

when importing serialized state, state is rewritten correctly when exporting state, serialized representation is correct