Add more error code support to GHC.Windows (patch)

This patch adds some infrastructure for the Windows I/O manager currently in progress. It organizes GHC.Windows a little, and adds some things:

• A LPWSTR type alias. I'm not a Windows expert, but I think we should avoid LPTSTR in our FFI declarations. It depends on whether or not the UNICODE macro is defined—a notion that makes little sense in a Haskell program. Using LPWSTR makes our intention clearer.

I left LPTSTR in for compatibility. Should we DEPRECATE it?

• c_maperrno_func. The existing c_maperrno calls GetLastError, converts the result, then sets errno. c_maperrno_func exposes the conversion as a pure function.

This helps eliminate a caveat in failWith, which expects GetLastError to return the error passed to failWith. This is not the case, for example, when throwing the error in another thread.

• failWith, failIf, ... error guards from System.Win32.Types.
• ErrCode, getLastError, getErrorMessage, errCodeToIOError: Lets you work with the system error code more directly.
• iNVALID_HANDLE_VALUE

Behavior change: throwGetLastError now attaches the system error message, making it (almost) equivalent to errorWin from System.Win32.Types. Moreover, it no longer has the side effect of setting errno, thanks to c_maperrno_func.

Trac metadata

Trac field	Value
Version	7.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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Blocking	#7353 (closed)
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