tightly pack vertices array by defining as 2D array instead of Vec of arrays

[elh]

get better data locality. hit my arbitrary goal of >500K permutations per second! decently satisfied with that. lets timebox some attempts to get to 1M per second

case	n	result	dur_ms	dur_pretty
F(2)	2	1	0	4.98µs
F(4)	4	5	0	8.63µs
F(6)	6	35	0	53.44µs
F(8)	8	319	0	656.73µs
F(10)	10	3559	6	6.37ms
F(12)	12	46841	79	79.95ms
F(14)	14	709601	1281	1.28s


F(16)	16	12156445	23447	23.45s