Fixed mysterious error message if start defined as a function

CHANGES

@@ -1,5 +1,9 @@
 Version 0.4
 -----------
+04/09/2019 Fixed very mysterious error message that resulted if you 
+           defined a grammar rule called "start".   start can now 
+           be a string or a function.
+
 04/09/2019 Minor refinement to the reporting of reduce/reduce conflicts.
            If a top grammar rule wasn't specified, SLY could fail with 
            a mysterious "unknown conflict" exception.  This should be

sly/yacc.py

@@ -486,6 +486,9 @@ class Grammar(object):
     # -----------------------------------------------------------------------------
 
     def set_start(self, start=None):
+        if callable(start):
+            start = start.__name__
+
         if not start:
             start = self.Productions[1].name