Rework colormaps, prepare contourplot

[t-bltg]

• Add more colormaps (8bit): rationale for this: enhanced colormap range using 8bit colors instead of arbitrary 24bit rgb, new reference files cover these. These new colormaps also prepare the change initiated at Rework structure for 8bit -> 24bit mode #202 for truecolor 24bit support.
• Change existing colormap data structure to use Vector{NTuple} instead of Vector{Vector}, enforce const.
• Move colorbar computations from heatmap to canvas in order to prepare contourplot integration, and fix [FR] stand-alone version of printcolorbarrow to draw a colorbar in isolation #169.
• Move colormap callback computation to colormaps.jl, added tests are covering this.

[codecov-commenter]

Codecov Report

Merging #203 (3b63f74) into master (5beb0b6) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #203      +/-   ##
==========================================
- Coverage   99.65%   99.65%   -0.01%     
==========================================
  Files          23       23              
  Lines        1171     1166       -5     
==========================================
- Hits         1167     1162       -5     
  Misses          4        4              

Impacted Files	Coverage Δ
src/canvas/heatmapcanvas.jl	96.15% <ø> (-2.13%)	⬇️
src/colormaps.jl	100.00% <ø> (ø)
src/canvas.jl	100.00% <100.00%> (ø)
src/interface/heatmap.jl	100.00% <100.00%> (ø)
src/plot.jl	100.00% <100.00%> (ø)

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[johnnychen94]

Looks good... One small suggestion and one question.

[t-bltg]

CI is finally looking green.