Write tests for pow, fix problems they find
Write tests for pow, fix problems they find

Specifically, the tests found problems in both raising things to the 0
power and in raising negative numbers to powers.

I also changed pow() to borrow rather than take ownership of its
argument, since having to use foo.clone.pow() seemed really weird to
me.

mbrubeck/ramp,kunerd/ramp,anfractuosity/ramp-new,huonw/ramp,Aatch/ramp