Add Send + Sync markers for AlignedVec and Plan.

[richard-uk1]

AlignedVec is definitely safe to be Send and Sync as it is mutable only behind unique mut pointers.

It is harder to reason about Plan being Sync because we don't know if it uses any UnsafeCell-like behaviour internally in fftw. However we can be fairly sure it is Send, as it will be used in situations that rely on using it in different threads (e.g. in a jack realtime thread).

Using this crate with jack is why I need the changes in this PR to use the library.

[richard-uk1]

friendly ping

[TallJimbo]

I saw this while contemplating opening my own PR (or at least an issue) to make the Plan execute operations take non-mut references to self, because the FFTW docs are pretty clear about plan execution being thread-safe (and not modifying any internal data) as long as the new-array interfaces are used (which is what the safe Rust interface always does). Unless there's some subtlety I've missed, I think that means Plan can be sync, too.

[hombit]

@termoshtt, can you merge it, please?

[hombit]

Thank you!

[termoshtt]

https://github.com/rust-math/fftw/releases/tag/fftw-0.7.0 released with #109