Got rid of LOW PRIORITY TODO
after deciding it would be better not to pursue it.
Very minor comment edits.
Used static_assert to force Decimal constructor-from-string to take
either std::string or std::wstring, rather than the existing vaguely
documented requirement that it be one of these types.
Improvements and tidy-ups in comments and documentation in decimal.hpp.
Got rid of weird error in Decimal comments.