Merge branch 'num2str' of https://github.com/bvanassche/fio

* 'num2str' of https://github.com/bvanassche/fio:
  num2str(): Add the E (exa) prefix
  Add a num2str() unit test
  num2str(): Fix overflow handling
  num2str(): Remove the fmt[] array
  num2str(): Use asprintf() instead of malloc()