os: remove os_mac.v and os_win.v, fix os.ls() on Windows

vlib/os/os_win.v

@@ -1,38 +0,0 @@
-// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
-// Use of this source code is governed by an MIT license
-// that can be found in the LICENSE file.
-
-module os
-
-pub fn ls(path string) []string {
-	mut res := []string
-	return res
-}
-
-pub fn getwd() string {
-	mut buffer := malloc(512)
-	
-	buffer = C._getcwd(0, 0)
-	// A NULL return value indicates an error
-	if isnil(buffer) {
-		return ''
-	}
-	return string(buffer)
-}
-
-const (
-	FILE_ATTRIBUTE_DIRECTORY = 16
-)
-
-fn is_dir(path string) bool {
-	val := int(C.GetFileAttributes(path.cstr()))
-	return val &FILE_ATTRIBUTE_DIRECTORY > 0
-}
-
-fn chdir(path string) {
-	C._chdir(path.cstr())
-}
-
-fn log(s string) {
-}
-