Have assertion macro and FATAL() text ask user to file a bug, provide URL to do so.

Switch to Assert() from assert() to make it clear it's not the C stdlib one we're
using any more.

stmt.h

@@ -341,7 +341,7 @@ public:
 
     Like print() above, since we don't have strings as first-class types in
     the language, we need to do some gymnastics to support it.  Like
-    assert() in C, assert checks the given condition and prints an error
+    assert() in C, assert() checks the given condition and prints an error
     and calls abort if the condition fails.  For varying conditions, the
     assert triggers if it's true for any of the program instances.
 */