Previously, clojure.lang.PersistentVector was not special-cased on writing
and reading. This resulted in fressian-encoded streams, containing Clojure
vectors, being coerced on reads into java.lang.ArrayList. Coercion here is
a bit surprising and potentially causes unexpected errors.
Here we explicitly mark clojure.lang.PersistentVector instances with the
vec tag on writes and upon reads decode these back to Clojure's vector type
by casting them with a call to vec.
The real problem here is that List extensibility is not exposed in Fressian itself. We would need to look into either
user specified handlers where FressianReader calls getHandler
documenting the ConvertList interface
making the list extension point more feel like the other collection types for customization
Needs Fressian enhancement instead.