multiprecision_int is a wrapper around boost::multiprecision::cpp_int that adds
no functionality. The intent is to allow forward declration; cpp_int is so
complicated that just finding out what its true type is a difficult exercise, as
it depends on many internal declarations.
Because cpp_int uses expression templates, the implementation has to explicitly
cast to the desired type in many places, otherwise the C++ compile is presented
with too many choices, especially in conjunction with data_value (which can
convert from many different types too).
