15921, 16064, 16067 Import CodingDirectives + related fixes

Fixes #15921, #16064, #16067 

This was actually quite some fun. Thanks to the implicit context now being carried by the RequestContext, I abandoned all of Spray's magnets in EncodingDirectives in the nearby park where they now roam trying to attract some easy prey... :rabbit: :whale: :fried_shrimp: 

/cc @sirthias
@jrudolph have you started the rebase? I gave it a try, but right now I'm stuck with this withFlowMaterializer. I'm not sure what was removed and not.