`lmul!` should be required for in-place application of plans

[devmotion]

This is basically the same issue/suggestion as https://github.com/JuliaMath/FFTW.jl/issues/77. In my opinion, it would make sense to apply in-place plans with lmul! already in the generic implementation of fft!, bfft!, and ifft! in https://github.com/JuliaMath/AbstractFFTs.jl/blob/master/src/definitions.jl#L50-L52.