"none" keyword for optionals + more memory fixes

vlib/builtin/map.v

@@ -155,8 +155,7 @@ fn (m map) bs(query string, start, end int, out voidptr) {
 fn preorder_keys(node &mapnode, keys mut []string, key_i int) int {
 	mut i := key_i
 	if !node.is_empty {
-		mut a := *keys
-		a[i] = node.key
+		keys[i] = node.key
 		i++
 	}
 	if !isnil(node.left) {

vlib/builtin/option.v

@@ -5,21 +5,26 @@
 module builtin
 
 struct Option {
-	data [255]byte 
-	error string
-	ok    bool
+	data     [255]byte
+	error    string
+	ok       bool
+	is_none  bool
 }
 
 // `fn foo() ?Foo { return foo }` => `fn foo() ?Foo { return opt_ok(foo); }`
 fn opt_ok(data voidptr, size int) Option {
 	if size >= 255 {
-		panic('option size too big: $size (max is 255), this is a temporary limit') 
-	} 
-	res := Option { 
+		panic('option size too big: $size (max is 255), this is a temporary limit')
+	}
+	res := Option {
 		ok: true
 	}
-	C.memcpy(res.data, data, size) 
-	return res 
+	C.memcpy(res.data, data, size)
+	return res
+}
+
+fn opt_none() Option {
+	return Option{ is_none: true }
 }
 
 pub fn error(s string) Option {