Not for merging: benchmarking generate infections

[sbfnk]

Two experiments:

1. Replace exponential with multiplications (exp)
2. Initialise vectors to zero and replace sums with assignments (zero) - enabled by cd2e738 (#366)

> summary %>% filter(name == "infections")
# A tibble: 3 × 7
  model name        mean   low  high lower higher
  <chr> <chr>      <dbl> <dbl> <dbl> <dbl>  <dbl>
1 main  infections  8.18  7.93  8.39  7.86   8.53
2 exp   infections  7.85  7.61  8.10  7.56   8.30
3 zero  infections  6.26  6.11  6.44  6.01   6.50

Both appear to lead to speed ups and, I think, would be good changes to implement.

[sbfnk]

Once #537 is merged this can just add the profiling bits added for further exploration.

[sbfnk]

Closing as superseded by #614