More optimizations

[timholy]

17 made steps to improve performance, but there was still more that could be done:

• hoist more invariant array-access out of loops
• precompute the inputval => outputval lookup table. This saves unnecessary arithmetic and convert operations
• skip a out .= img, which just added an unnecessary trip through both arrays
• use @inbounds in a few more places

The aggregate impact: on master,

julia> using ImageContrastAdjustment, BenchmarkTools

julia> img = rand(Float32, 2048, 2048);

julia> @btime adjust_histogram($img, Equalization(nbins = 256));
  44.144 ms (6 allocations: 16.01 MiB)

but on this branch

julia> @btime adjust_histogram($img, Equalization(nbins = 256));
  18.238 ms (7 allocations: 16.01 MiB)

[codecov[bot]]

Codecov Report

Merging #53 (776b4e4) into master (37000d7) will decrease coverage by 0.07%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage   95.97%   95.89%   -0.08%     
==========================================
  Files          12       12              
  Lines         546      585      +39     
==========================================
+ Hits          524      561      +37     
- Misses         22       24       +2     

Impacted Files	Coverage Δ
src/algorithms/common.jl	100.00% <100.00%> (ø)
src/algorithms/equalization.jl	93.75% <100.00%> (+0.41%)	:arrow_up:
src/build_histogram.jl	92.00% <100.00%> (-1.48%)	:arrow_down:
src/algorithms/linear_stretching.jl	95.00% <0.00%> (-1.62%)	:arrow_down:
src/algorithms/adaptive_equalization.jl	100.00% <0.00%> (ø)
src/algorithms/midway_equalization.jl	96.15% <0.00%> (+0.07%)	:arrow_up:
src/algorithms/gamma_correction.jl	92.59% <0.00%> (+0.59%)	:arrow_up:
src/core.jl	95.65% <0.00%> (+0.65%)	:arrow_up:
... and 1 more

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