Try to determine if `c` is in the Mandelbrot set, using at most `limit` iterations to decide. If `c` is not a member, return `Some(i)`, where `i` is the number of iterations it took for `c` to leave the circle of radius two centered on the origin. If `c` seems to be a member (more precisely, if we reached the iteration limit without being able to prove that `c` is not a member), return `None`.
(c: Complex<f64>, limit: usize)
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