Add operations for obtaining operating-system handles

[jeltsch]

In order to decouple base versioning from GHC versioning, it is necessary to remove various GHC-specific modules from base. Among these module are GHC.IO.Handle.Types and GHC.IO.Handle.Internals. Currently, there are various packages on Hackage that use these modules. One purpose of such use is obtaining operating-system handles (file descriptors, Windows handles) that underlie Haskell handles.

GHC merge request !14732 adds a new module System.IO.OS to base, which contains several operations for obtaining operating-system handles. These new operations, together with the operation withHandleToHANDLE from the Win32 package, have been used to implement alternatives to operating-system handle acquisition based on the above-mentioned internal modules in a total of six packages on Hackage. For details, see the following respective pull requests:

• UnkindPartition/ansi-terminal#187
• haskell/haskeline#211
• jeltsch/hsshellscript#2
• haskell/process#348
• Happstack/sendfile#5
• biegunka/terminal-size#20