util fragment_range: add general linearisation functions
util fragment_range: add general linearisation functions

All FragmentRange implementations can be linearised in the same way, so
let's provide linearized() and with_linearized() functions for all of
them.

scylladb/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla