`plan_rfft` eltype should be `Complex`

This caught me by surprise:
```julia
julia> x = randn(5); P = plan_rfft(x);

julia> eltype(P)
Float64

julia> P * x
3-element Vector{ComplexF64}:
-0.3514539021978911 + 0.0im
-0.8023047651675712 + 1.243731722698685im
2.8759555200391196 - 2.679171588261927im
```
If a "plan" is in some sense analogous to a matrix the `eltype` should be type of the output, in this case `ComplexF64`.