[Cfp-interest 2899] incorrect definitions of *_DECIMAL_DIG and DECIMAL_DIG

[Vincent Lefevre]

The *_DECIMAL_DIG macros are defined as follows:

  number of decimal digits, n, such that any floating-point number
  with p radix b digits can be rounded to a floating-point number
  with n decimal digits and back again without change to the value,
  [...]

However, this is true only if rounding to nearest is used for these
roundings. Ditto for the DECIMAL_DIG macro.

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Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
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