win32.c: convert by Win32 API

* win32/win32.c (win32_direct_conv, rb_w32_readdir): convert UTF-8
  and filesystem code page by using Win32 API directly.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48644 b2dd03c8-39d4-4d8f-98ff-823fe69b080e