Support custom `persist` and `hydrate` callbacks for primitive types.

[solid-yuriiprykhodko]

As pointed out by @solid-vovabeloded: Currently if we use HydratedSubject with a primitive type, custom persist and hydrate callbacks won't be invoked, or used at all:

The idea is to change the check order in the above method to use the custom callbacks, if they were provided, and then fall back to standard handling.