Remove dependency on tokio

[chris-srlabs]

Alternative implementation of #183 . This is faster in case of multiple seeds (n=1800), a test on my machine showed

# PR 183
Benchmark 1: cargo ziggy run -t 10
  Time (mean ± σ):     24.181 s ±  0.148 s    [User: 6.290 s, System: 2.029 s]
  Range (min … max):   23.938 s … 24.405 s    10 runs
 
# THIS PR
Benchmark 1: cargo ziggy run -t 10
  Time (mean ± σ):      7.968 s ±  0.262 s    [User: 6.056 s, System: 1.906 s]
  Range (min … max):    7.660 s …  8.487 s    10 runs