ivm: parallel evaluator

[tjjfvi]

• Resolves ivm: parallel evaluator #4

mandelbrot_tga.vi, on a machine with 12 performance cores + 4 efficiency cores:

Threads	Time	Speedup	Efficiency
1	13.358	1.00	100.00
2	7.169	1.86	93.17
3	4.967	2.69	89.64
4	3.768	3.55	88.63
5	3.063	4.36	87.22
6	2.649	5.04	84.04
7	2.214	6.03	86.19
8	1.928	6.93	86.61
10	1.544	8.65	86.52
12	1.269	10.53	87.72
12+4	1.105	12.09	75.55