Support new AbstractFFTs.AdjointStyle trait for FFT and DCT plans

[gaurav-arya]

Supports the new AbstractFFTs.AdjointStyle trait. This is a draft PR because JuliaMath/AbstractFFTs.jl#78 and JuliaMath/AbstractFFTs.jl#109 are not merged yet, which this PR relies on.

[vpuri3]

@gaurav-arya where is testbackend.jl?

[gaurav-arya]

I think you can remove that: it was one attempt at reusing AbstractFFTs' tests here but we haven't yet agreed on a good solution, see JuliaMath/AbstractFFTs.jl#78.

If you'd like to push this through perhaps if you can think of a minimal self contained test here to test the traits, we could sidestep the problem and get this merged (note I don't have merge rights, so ultimately the decision should be verified with someone who does).

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +2.08% 🎉

Comparison is base (008bc5b) 70.99% compared to head (969315a) 73.08%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
+ Coverage   70.99%   73.08%   +2.08%     
==========================================
  Files           5        5              
  Lines         531      535       +4     
==========================================
+ Hits          377      391      +14     
+ Misses        154      144      -10     

Files Changed	Coverage Δ
src/dct.jl	91.66% <100.00%> (+10.81%)	⬆️
src/fft.jl	70.56% <100.00%> (+1.38%)	⬆️

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[gaurav-arya]

This is ready!

[gaurav-arya]

Bump

[stevengj]

LGTM.

[gaurav-arya]

@stevengj could FFTW get a release for this PR? It would be helpful for including FFTW's run of the test suite in downstream tests for JuliaMath/AbstractFFTs.jl#118