Get rid of EExplained.

Use a dummy info::info function that is intercepted by the type-checker and
allows one to dump information about a variable.

Use

  info::info x;

if you want to know what permissions are available for x at a given program
point.

If calling Mezzo with "-explain x11", enjoy the beautiful graphviz output. If
calling Mezzo with "-explain html", enjoy the beautiful, clickable html output.