mostly complete numeric_limits<>

[Joe Buck]

> We have to decide where num_limits generator will live
> and what part of full header should be generated.

I think that the process should look a lot like the "enquire"
program, which generates float.h for ANSI C by doing tests on
the machine.  (But there are problems with cross-compilation:
how is this handled by enquire?).

Another approach is just to extend enquire a bit to make the extra
information.