5779.feat

By blocks ("... by { ... }") are now available for assert statements, call statements, and the three types of assignments (:=, :-, :|). Also, by blocks should now be more intuitive since they enable proving any assertions on the left-hand side of the 'by', not just the 'outermost' one. For example, previously `assert 3 / x == 1 by { assume x == 3; }` could still give a possible division by zero error, but now it won't.