Enum of transforms

[jishnub]

Instead of using a number of consts, we may use an enum. This doesn't change any behavior, but makes the types more readable.

julia> f = FastTransforms.plan_cheb2jac(rand(10), 0, 0);

julia> typeof(f)
FastTransforms.FTPlan{Float64, 1, FastTransforms.CHEB2JAC}

Should be non-breaking. I've checked locally that the tests for ApproxFun pass with this PR.

[codecov]

Codecov Report

Merging #181 (d021a4e) into master (8a82d79) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
- Coverage   78.79%   78.75%   -0.04%     
==========================================
  Files          13       13              
  Lines        2108     2109       +1     
==========================================
  Hits         1661     1661              
- Misses        447      448       +1     

Impacted Files	Coverage Δ
src/libfasttransforms.jl	77.81% <0.00%> (-0.14%)	⬇️

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[MikaelSlevinsky]

For historical reference, this structure was taken from FFTW.jl, https://github.com/JuliaMath/FFTW.jl/blob/17bc81a0fcf9875d777ea4bee2fca70fc23c8a0c/src/fft.jl#L81-#L99.