Prepare autosanitization

See my sandbox for a background, links and proof of concept code.
According to the berkeley paper linked there proper (context stack aware) autosanitization would be a USP in the FOSS market.

Of course no chance to get it complete in D8 core but hopefully a chance to prepare it to be done in contrib.
I have prepared the technical basics but the research what our twig implementation needs for this is beyound my insight.

First step agreed with chx: #1751486: Add context to TemplateData class