sharded: map: elide copy of mapper result into vector
sharded: map: elide copy of mapper result into vector

It doesn't matter if the result is trivial
but if it isn't, move the (rvalue) result rather
than copying it.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

avikivity/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar