reduce ssvWriter allocation in funcSort

mark             old ns/op     new ns/op     delta
BenchmarkFuncSort     1572          1413          -10.11%

benchmark             old allocs     new allocs     delta
BenchmarkFuncSort     8              7              -12.50%

benchmark             old bytes     new bytes     delta
benchmarkFuncSort     188           156           -17.02%
2 files changed