test: add prime sum bench mark

[danvk]

Another performance test that's not so stack intensive as Fibonacci (#37).

Counting the primes less than 10,000:

• gravlax: 36.7s
• jlox: 11s
• clox: 3.6s

[github-actions]

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[danvk]

@all-contributors please add @danvk for test.

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