support for empty packages (resolves #9)

src/Language/SystemVerilog/Parser/Parse.y

@@ -570,7 +570,7 @@ DefparamAsgn :: { (LHS, Expr) }
   : LHS "=" Expr { ($1, $3) }
 
 PackageItems :: { [PackageItem] }
-  : PackageItem              { $1 }
+  : {- empty -}              { [] }
   | PackageItems PackageItem { $1 ++ $2 }
 PackageItem :: { [PackageItem] }
   : DeclTokens(";")    { map Decl $ parseDTsAsDecls $1 }

test/basic/empty_constructs.sv

@@ -0,0 +1,8 @@
+task fun;
+endtask
+function fun;
+endfunction
+package pop;
+endpackage
+module top;
+endmodule

test/basic/empty_constructs.v

@@ -0,0 +1,2 @@
+module top;
+endmodule