`size`, `length`, `ndims` for `Plan` inconsistent with Base usage

Ref. https://github.com/JuliaMath/FFTW.jl/issues/159

While it is certainly useful to be able to query a `Plan` for the size of the input data it expects, using `size` for this seems wrong. It should give the dimensions of the `Plan` itself, whatever that might be. As noted in https://github.com/JuliaMath/FFTW.jl/issues/159, a `Plan` for 1d input of length `n` could be considered equivalent to a matrix with size `(n, n)`, but `size` would just return `n`.

So we might consider renaming `size` as used here, and maybe `ndims` along with it if deemed necessary to have it, noting the trivial definition `length(size(p))`. I don't see how `length` is useful, so that could maybe just be deleted.

I'm skeptical about defining `size`, `length`, `ndims` in a way (more) consistent with Base, unless we make `Plan`s iterable (and maybe even indexable), where I don't see much benefit.