winfsp-rs 0.12.0

• Build against WinFSP 2.1 headers
• Use windows 0.61 internally
• Allow choosing external version of windows crate via feature
  • windows-56, windows-60, windows-61, windows-62 are supported
• Fix memory leaks by allowing FileSystemHost to be dropped (#38 by @syborg64)
  • The FileSystemService user context pointer now has an implicit lock to accommodate this soundly. This will only affect users using the with_control callback, which should have been keeping a lock internally regardless for safety.
• Cross-compile winfsp-sys (#42 by @LinusChen-yf )
• Make service callbacks return FspError (#43 by @LinusChen-yf )