compiler: allow function expecting a void* to receive a byteptr

vlib/builtin/isnil_test.v

@@ -0,0 +1,19 @@
+fn test_isnil_byteptr(){
+	pb := byteptr(0)
+	assert isnil( pb )
+}
+
+fn test_isnil_voidptr(){
+	pv := voidptr(0)
+	assert isnil( pv )
+}
+
+fn test_isnil_charptr(){
+	pc := &char(0)
+	assert isnil( pc )
+}
+
+fn test_isnil_intptr(){
+	pi := &int(0)
+	assert isnil( pi )
+}