FFT demos

These demonstrate the functionality of the FFT and IFFTs.  I'm working
on the general functions as we speak; these both include some arrays of
random numbers in them for testing/demonstration purposes.
Per Doctor Mandel's recommendations, the code is significantly more
robust and reliant on variables rather than random, absolute
declarations.

I've gotten the FFT to reproduce Matlab outputs perfectly, as well as
recover the original values using the IFFT.  There are two key steps
that differentiate the Matlab outputs from the 'raw' DSP FFT outputs:
FIRST the output is bit-reversed (using bit_rev function, see the
functions themselves for detail).
SECONDLY the sign of the imaginary components is switched.
In the IFFT, I do these things in the reverse order: switch signs first,
then do bit-reversal, then run it through the actual DSP IFFT.