Convert direction and flag vars to constants
Remove the superfluous Direction and Flag types
Move destroyPlan() into an anonymous function
Remove Alloc*() functions

The reflect.SliceHeader type is labeled as non-portable and I haven't
thought of a way to construct the slices without it.
Remove ExecuteNewArray()

The FFTW manual recommends against using the new-array execution
interface.
Remove *[23]d() functions

The *[23]d() functions assume that the slice-of-slices all point
to a single, contiguous array as backing storage, but this isn't
verified. If it isn't true, FFTW will access garbage instead.
Check arguments to and return values from FFTW
Add doc comments
Add missing planner flags
Shorten up references to Plans
Merge branch 'cleanup'