make_literal is losing some precision when making literal floating
point values because it uses digits10, but that only gives us the
number of decimal digits that will survive a decimal->native>decimal
conversion; what we are doing is a native->decimal->native conversion,
which requires the user of ::max_digits10 (which is 2 (double) or 3
(float) larger than ::digits10).
max_digits10 is a c++11 feature, however, so this commit uses
digits10 + 3 when the c++11 numeric_limits isn't available.
