Avoid vector copy in NTT call

Perform in-place NTT in SwitchFormat(), yielding ~1% slowdown in SwitchFormat().

Running:

OMP_NUM_THREADS=1 ./bin/benchmark/poly-benchmark-16k --benchmark_filter="DCRT_ntt" --benchmark_min_time=10

yields

Before:
DCRT_ntt/towers:1       1375 us         1374 us        10186
DCRT_ntt/towers:2       2761 us         2748 us         5096
DCRT_ntt/towers:4       5521 us         5509 us         2540
DCRT_ntt/towers:8      11043 us        11031 us         1268


After:
DCRT_ntt/towers:1       1394 us         1391 us        10063
DCRT_ntt/towers:2       2782 us         2780 us         5036
DCRT_ntt/towers:4       5593 us         5568 us         2515
DCRT_ntt/towers:8      11181 us        11149 us         1255