README: update
bigmem: improve bigmem cache

Improve time to use the bigmem cache by 50%.

- Use a new alogrithm to compute the index of the array. This algorithm
  use the shift operator (<<) over the std::pow.

- Remove the pimpl idiom.
main: add color to the stress result

The minimum and maximum time to resolve the random stress are show with
red and yellow colors.