Better NTT & invNTT codes

[fionser]

1. Re-ordering inv_root_power so that the access pattern in invNTT can be sequential
2. replace the div_two trick in invNTT. I found merge the n^{-1} step into the last layer of invNTT is faster.

Performance tests used the below parameters.

/
| Encryption parameters :
|   scheme: BFV
|   poly_modulus_degree: 16384
|   coeff_modulus size: 438 (48 + 48 + 48 + 49 + 49 + 49 + 49 + 49 + 49) bits
|   plain_modulus: 786433
\

Before:

Average multiply: 70728 microseconds
Average multiply plain: 11047 microseconds
Average square: 50622 microseconds
Average relinearize: 26351 microseconds
Average rotate rows one step: 26760 microseconds
Average rotate rows random: 118160 microseconds
Average rotate columns: 27619 microseconds

After:

Average multiply: 65593 microseconds
Average multiply plain: 10108 microseconds
Average square: 46441 microseconds
Average relinearize: 25216 microseconds
Average rotate rows one step: 25467 microseconds
Average rotate rows random: 119536 microseconds
Average rotate columns: 25547 microseconds