Re-organize modules

[dakom]

Should we organize the modules differently? Maybe re-export everything in a top-level SodiumFRP ?

Personally I don't mind the current setup, but it does lead to multiple imports that could be avoided...

[clinuxrulz]

Only if there are exported functions/objects we do not wish the end-user to have that are used internally. If we do not need package private, then we can leave it as is.