I thought it would be useful to have an issue to track functions/filters that themers miss when working on the conversion and perhaps later on.
And just another thought: we will probably need a way to let other modules provide additional functions/filters. But we can save that for later.
We'll need to decide weather we want things like t or count to be filters or functions in twig.
Drupal functions to Twig functions
PHP functions to Twig functions
Drupal functions to Twig filters
|withoutfilter. @see https://www.drupal.org/node/2212845