Increase VpMult batch size

[tompng]

The bottleneck was the number of divmod operation which depends on batch size.
Larger batch size is always better.

batch_size*BASE**2 doesn't need to be less than 2**63-1, it needs to be less than 2**64. We can increase batch size from 8 to 16.

50-350 words multiplication gets about 1.8 times faster

Benchmark

Multiplication time of x*x where x = BigDecimal('999999999'*words). Smaller is better.

words	time(v4.1.0)	time(master)	time(this pull request)	memo
1	0.09 ns	0.06 ns	0.06 ns
5	0.18 ns	0.08 ns	0.08 ns
10	0.53 ns	0.17 ns	0.13 ns
50	11.73 ns	2.54 ns	1.50 ns
100	26.99 ns	9.86 ns	5.59 ns	v4.1.0 uses NTT mult
200	53.94 ns	37.77 ns	20.96 ns
300	109.03 ns	81.54 ns	46.07 ns
350	110.75 ns	110.43 ns	62.17 ns	master uses NTT mult
400	111.62 ns	112.43 ns	80.38 ns
450	113.02 ns	111.27 ns	111.12 ns	this pull uses NTT mult