Missing in-place real `rfft!` interface

Hi,

since there is a [PR](https://github.com/JuliaMath/FFTW.jl/pull/222) for `rfft!` in FFTW we probably should also add `rfft!` in AbstractFFTs?

In general [in-place real-to-complex FFT](https://docs.nvidia.com/cuda/cufft/index.html#data-layout) seems to be supported by CuFFT as well, right?

I recall that I was discussing this somewhere with @maleadt but I can't remember exactly.

What do you think about that?

Best,

Felix