conversion for function void (resolves #32)

test/basic/function_void.sv

@@ -0,0 +1,6 @@
+module top;
+    function void foo;
+        $display("foo called");
+    endfunction
+    initial foo;
+endmodule

test/basic/function_void.v

@@ -0,0 +1,6 @@
+module top;
+    task foo;
+        $display("foo called");
+    endtask
+    initial foo;
+endmodule