setting mpfr precision uses wrong precision and trashes value

[claude@…]

​http://svn.boost.org/svn/boost/trunk/boost/multiprecision/mpfr.hpp Two bugs in boost/multiprecision/mpfr.hpp:920-923

1. set_prec trashes the value, should use prec_round
2. precision is converted from bits to digits10, should be from digits10 to bits

void precision(unsigned digits10) BOOST_NOEXCEPT
{
  mpfr_set_prec(this->m_data, multiprecision::detail::digits2_2_10((digits10)));
}

should be more like

void precision(unsigned digits10) BOOST_NOEXCEPT
{
  mpfr_prec_round(this->m_data, multiprecision::detail::digits10_2_2((digits10)), GMP_RNDN);
}

[John Maddock]

(In [84763]) Apply patch from #8692. Refs #8692.

[John Maddock]

(In [84799]) Merge fixes for from Trunk. Fixes #8692. Fixes #8670. Fixes #8667.