Add some precompiles

[timholy]

On the current release, the first call looks like this:

julia> @time imfilter(img, KernelFactors.gaussian((3,3)));
  2.135609 seconds (3.46 M allocations: 211.109 MiB, 12.01% gc time, 94.98% compilation time)

julia> @time mapwindow(extrema, img, (3,3))
  0.756083 seconds (1.17 M allocations: 68.258 MiB, 6.57% gc time, 99.89% compilation time)

On this branch, it looks like this:

julia> @time imfilter(img, KernelFactors.gaussian((3,3)));
  0.620004 seconds (362.78 k allocations: 40.455 MiB, 1.47% gc time, 96.09% compilation time)

julia> @time mapwindow(extrema, img, (3,3))
  0.385470 seconds (334.52 k allocations: 19.585 MiB, 9.43% gc time, 99.82% compilation time)

Quite a nice reduction in latency.

I should note this requires https://github.com/JuliaMath/AbstractFFTs.jl/pull/48 and also some changes to FFTW.jl that I haven't yet submitted since they rely on the changes in AbstractFFTs.

[codecov[bot]]

Codecov Report

Merging #201 (4ca2542) into master (83ac3ed) will decrease coverage by 0.58%. The diff coverage is 95.83%.

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
- Coverage   91.59%   91.00%   -0.59%     
==========================================
  Files           9       10       +1     
  Lines        1380     1412      +32     
==========================================
+ Hits         1264     1285      +21     
- Misses        116      127      +11     

Impacted Files	Coverage Δ
src/ImageFiltering.jl	77.27% <ø> (ø)
src/kernelfactors.jl	91.15% <90.00%> (-2.42%)	:arrow_down:
src/precompile.jl	94.11% <94.11%> (ø)
src/border.jl	93.02% <100.00%> (-1.13%)	:arrow_down:
src/kernel.jl	98.40% <100.00%> (-1.60%)	:arrow_down:
src/mapwindow.jl	86.22% <100.00%> (-0.45%)	:arrow_down:
src/utils.jl	87.30% <0.00%> (-1.59%)	:arrow_down:
... and 2 more

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[timholy]

I think we can merge this as-is, and continue in separate efforts.