Add support for multi-threaded plan execution

[SallySoul]

Possible dupe of #126

Before I dive in, I want say I'm interested in opening a PR for this. I was hoping to hear if anyone had looked into this before, or you had any feedback.

For reference, the FFTW3-multithreaded docs are here

My initial thoughts on what needs to happen:

• This should be an opt in feature that doesn't change the default behavior of the fftw crate.
• Add a new crate feature to fftw-src that builds fftw3 with thread support, and links library
  • Could add feature for OpenMP support too, but thats not a priority for me at the moment.
  • Unclear how this would work on windows, also not a priority for me, but I do have a windows machine I can experiment on.
• Add a new crate feature to fftw-sys that would export fftw_init_threads and fftw_plan_with_nthreads
• Add a new create feature to fftw that when enabled adds new planner constructors that accept a "# of threads" parameter
  • The old constructors can be assumed to use 1 thread
  • fftw_init_threads would also need a counterpart somewhere.