map/reduce/filter functions

The map() and reduce() functions in the core library are the expected functional programming primitives. But filter() is not, it's an FFTW function instead. I'd recommend renaming the FFTW function as something like ffilter() or vfilter(), and defining filter(f, x) to be the equivalent of something like x[f(x)], where x is an iterable and f() returns a boolean. 
