mutation_partition: Shuffle declarations
mutation_partition: Shuffle declarations

Its methods that provide access to enclosed collections of rows
and range tombstones are intermixed, so group them for smoother
next patching and mark noexcept while at it.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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