
sprintf_double using frexp & pow:
  convert 300000 doubles with sprintf_double: 0.111671
  convert 300000 doubles with sprintf: 0.450840
  but I get many failures at the last digit (nearly all fail for doubles)

sprintf_double using log10 & pow:
  convert 300000 doubles with sprintf_double: 0.133986
  convert 300000 doubles with sprintf: 0.459899
  I get only 35 / 1000 failures

