Provide `rfft!` and `irfft!` through a submodule

Is it desirable to have a submodule living in FFTW.jl to provide the inplace real-to-complex and complex-to-real transformations?

The functionality is already coded in [InpalceRealFFT.jl](https://github.com/favba/InplaceRealFFT.jl) and could remain living as a separate package. Some people [expressed the desire](https://discourse.julialang.org/t/ann-inplacerealfft-jl-inplace-real-to-complex-and-complex-to-real-fft/8116/2) to have it in FFTW.jl
Basically, a `PaddedArray` type is created to deal with proper padding of the real data, and methods `rfft!`,`irff!`,`brfft!`,`plan_rfft!`,`plan_irfft!` and `plan_brfft!` are created to work with `AbstractPaddedArray`s, an interface to provide custom PaddedArray types.

I could make a PR if this is desirable.