more uniform Future.map: always internalize failure; added Future.flat as fast-path operation;
more uniform Future.map: always internalize failure;
added Future.flat as fast-path operation;

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle