[Nickle]A usable calculator again

[Keith Packard]

With much typing, I've replaced the old machine-specific floating point 
code with arbitrary precision stuff; this is useful, but it temporarily 
reduced the utility of nickle as a calculator -- the C math library can no 
longer be used.  

So I wrote an arbitrary precision math library in nickle itself; with
Bart's code to load a startup file when nickle launches, we'll be back to
where we were a week ago, except that there are still a lot of functions to
implement.

I've got sqrt, cbrt, exp, log, log10, sin, cos, sin_cos, tan, arcsin,
arccos, arctan and arctan2.  I'll have to do a bit of work to get the '^'
operator to use the nickle library code, but I think that's worth the
effort rather than recoding exp/log in C.

-keith