Rasterization in DevUtils`

[taliesinb]

Changes

• This change depends on PR DevUtils: inplace builds, caching of libraries, hot reloading #529, since the code lives in DevUtils`
• This replaces Rasterization functions to produce inputs & outputs #506, which is considerably more complicated
• It introduces a single function, ExportPreviousInputOutputToMarkdown, which rasterizes an input+output pair, and produces markdown + image as a result. If possible the input is pure Mathematica, while the output is an image.

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This change is 

[taliesinb]

This is roughly how it works:

[taliesinb]

I have now documented the workflow in Contributing.