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

Overall, I like the overriding of `*` for planned transforms. But it is a bit odd that `*` modifies what you multiply in the following:
```julia
P = plan_fft!(x); P * x
```
I would propose deprecating `plan_fft!`, and instead only have `plan_fft` with the following usage for the in-place variants:
```julia
P = plan_fft(x); lmul!(P, x)
```