Happy should error out on duplicate productions

[harpocrates]

1. Consider the following short grammar:

   foo :: { Int }
      : int             { $1 }
   
   foo :: { Int }
      : int             { $1 + 4 }
   

   Happy should not accept this - it should complain that I've defined foo twice. Right now, it just silently picks one of the two.

2. Consider the following shorter grammar:

   foo :: { Int }
   foob
      : int             { $1 }
   

   Happy should not accept this - it should complain that the name foob does not match the expected name foo. Right now, it just uses foo and ignores foob completely.