Jakob accidentally undid some of my changes in commit aad4592, reverted that;
made style changes in multiple files;
in types/sigma: finished porting Coq-HoTT, and finished unfinished proof;
in axioms/funext: rename path_forall, make arguments implicit and make instance visible
Everything directly about truncations in the basic truncation library is ported.
Some theorems about other structures still need to be ported.
Also made some minor changes in hott.equiv
