Boost Review: Allow implicit conversions for all conversions which do not lose precision

[jzmaddock]

specifically all of the following should hold:

   static_assert(std::is_convertible<boost::decimal::decimal32, boost::decimal::decimal64>::value);
   static_assert(std::is_convertible<boost::decimal::decimal32, boost::decimal::decimal128>::value);
   static_assert(std::is_convertible<boost::decimal::decimal32, boost::decimal::decimal32_fast>::value);
   static_assert(std::is_convertible<boost::decimal::decimal32, boost::decimal::decimal64_fast>::value);
   static_assert(std::is_convertible<boost::decimal::decimal32, boost::decimal::decimal128_fast>::value);

   static_assert(std::is_convertible<boost::decimal::decimal64, boost::decimal::decimal128>::value);
   static_assert(std::is_convertible<boost::decimal::decimal64, boost::decimal::decimal64_fast>::value);
   static_assert(std::is_convertible<boost::decimal::decimal64, boost::decimal::decimal128_fast>::value);

   static_assert(std::is_convertible<boost::decimal::decimal128, boost::decimal::decimal128_fast>::value);

   static_assert(std::is_convertible<boost::decimal::decimal32_fast, boost::decimal::decimal64_fast>::value);
   static_assert(std::is_convertible<boost::decimal::decimal32_fast, boost::decimal::decimal128_fast>::value);
   static_assert(std::is_convertible<boost::decimal::decimal32_fast, boost::decimal::decimal32>::value);
   static_assert(std::is_convertible<boost::decimal::decimal32_fast, boost::decimal::decimal64>::value);
   static_assert(std::is_convertible<boost::decimal::decimal32_fast, boost::decimal::decimal128>::value);

   static_assert(std::is_convertible<boost::decimal::decimal64_fast, boost::decimal::decimal128_fast>::value);
   static_assert(std::is_convertible<boost::decimal::decimal64_fast, boost::decimal::decimal64>::value);
   static_assert(std::is_convertible<boost::decimal::decimal64_fast, boost::decimal::decimal128>::value);

   static_assert(std::is_convertible<boost::decimal::decimal128_fast, boost::decimal::decimal128>::value);

[mborland]

See: #816. Compile fail tests have been added for anything illegal and tests with results for all legal conversions as well.