Add Tex code generator class.
Convert XLabelRenderer to use new Tex generator.
Tex#comment and Tex#label now handle non-string text arguments.
Convert Tex from DSL to an object.

It was cumbersome to reference the containing object's 'self' from
within the DSL's do block.
Absorb XLabelRenderer into XLabel#render.
Convert ChartItemRenderer#render to use Tex.
Absorb ChartItemRenderer into CharItem#render.
Convert SeparatorItemRenderer#render to use Tex.
Absorb SepratorItemRenderer into SeparatorItem#render.
Move Tex instance creation up into TeXGenerator (WIP).
Remove BarXCoodinates#mid_point and #width.

These calculations are now the responsibility of the Tex class.
Add Tex#newline.
Replace FrameRenderer with Frame.
Add Chart#frame.
Absorb TeXGenerator into Chart#render.
Add Tex#echo.
Refactor: pass Tex instance into Chart#render.
Remorder methods in Tex.
Move knowledge of Tex from main down into Chart.
Chart#render now returns TeX code as a string.

The caller is responsible for deciding what to do with the string.