[libstdc++] preliminary headers supporting decimal float in C++

[Benjamin Kosnik]

> > include/tr24733/decimal
> > include/tr24733/decimal.h -- just C stuff if needed, ie conversions
> > to generic floating
> > include/tr24733/decimal_operators.h -- all the overloads.
> > 
> Once again, thanks for the advice.

No worries. Exact name here at your discretion (tr247733? trdfp?), just
document how to use it.

-benjamin