sstables: convert parse() for a self-describing type to concept
sstables: convert parse() for a self-describing type to concept

This parse() overload uses "not integral and not enum" to reject
non-self-describing types. Express it directly with the self_describing
concept instead.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla