implement numerator and denominator
fix math ffi on linux

On Linux, libm (and other libraries) are separate, so we have to use
different libraries. One of these is libm, which contains things like
`floor`, `atan2` and quite a few other things.

There should be symlinks from `libm` et al to `libSystem` on Mac [1], so
it should still work there.

[1]: http://www.finkproject.org/doc/porting/porting.en.html#basics.libraries
implement `int` and `float`
Merge branch 'num-improvements'

Conflicts:
	pixie/stdlib.pxi