Rework structure for 8bit -> 24bit mode

[t-bltg]

• use UInt32 instead of UInt8 for canvas color (allows packing r, g, b channels for truecolor support), blending rgb, ...

• use more Crayons objects, reduces test files by about 25%

• toggle between 24bit and 8bit at runtime

• benchmark for any regressions

[codecov-commenter]

Codecov Report

Merging #202 (bb0676e) into master (65b9368) will decrease coverage by 1.55%.
The diff coverage is 86.80%.

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
- Coverage   99.82%   98.27%   -1.56%     
==========================================
  Files          23       23              
  Lines        1164     1218      +54     
==========================================
+ Hits         1162     1197      +35     
- Misses          2       21      +19     

Impacted Files	Coverage Δ
src/graphics/boxgraphics.jl	100.00% <ø> (ø)
src/interface/heatmap.jl	100.00% <ø> (ø)
src/common.jl	86.89% <73.61%> (-13.11%)	⬇️
src/canvas/asciicanvas.jl	100.00% <100.00%> (ø)
src/canvas/blockcanvas.jl	100.00% <100.00%> (ø)
src/canvas/braillecanvas.jl	98.07% <100.00%> (ø)
src/canvas/densitycanvas.jl	100.00% <100.00%> (ø)
src/canvas/dotcanvas.jl	100.00% <100.00%> (ø)
src/canvas/heatmapcanvas.jl	98.24% <100.00%> (-0.04%)	⬇️
src/canvas/lookupcanvas.jl	100.00% <100.00%> (ø)
... and 6 more

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[t-bltg]

Superseded by #238.