Avoid errors on non UTF-8 Windows (#566)

* Avoid errors on non UTF-8 Windows

Problem
====

haddock exits with errors like below:

`(1)`

```
haddock: internal error: <stderr>: hPutChar: invalid argument (invalid character)
```

`(2)`

```
haddock: internal error: Language\Haskell\HsColour\Anchors.hs: hGetContents: invalid argument (invalid byte sequence)
```

`(1)` is caused by printing [the "bullet" character](http://www.fileformat.info/info/unicode/char/2022/index.htm) onto stderr.
For example, this warning contains it:

```
Language\Haskell\HsColour\ANSI.hs:62:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘toEnum’
    • In the instance declaration for ‘Enum Highlight’
```

`(2)` is caused when the input file of `readFile` contains some Unicode characters.
In the case above, '⇒' is the cause.

Environment
----

OS: Windows 10
haddock: 2.17.3
GHC: 8.0.1

Solution
====

Add `hSetEncoding handle utf8` to avoid the errors.

Note
====

- I found the detailed causes by these changes for debugging:
    - https://github.com/haskell/haddock/commit/8f29edb6b02691c1cf4c479f6c6f3f922b35a55b
    - https://github.com/haskell/haddock/commit/1dd23bf2065a1e1f2c14d0f4abd847c906b4ecb4
- These errors happen even after executing `chcp 65001` on the console.
  According to the debug code, `hGetEncoding stderr` returns `CP932` regardless of the console encoding.

* Avoid 'internal error: <stderr>: hPutChar: invalid argument (invalid character)' non UTF-8 Windows

Better solution for 59411754a6db41d17820733c076e6a72bcdbd82b's (1)

haddock-api/src/Haddock/Interface.hs

+{-# LANGUAGE CPP #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Haddock.Interface
@@ -48,14 +49,20 @@ import qualified Data.Set as Set
 import Distribution.Verbosity
 import System.Directory
 import System.FilePath
+import System.IO
 import Text.Printf
 
 import Digraph
 import DynFlags hiding (verbosity)
 import Exception
 import GHC hiding (verbosity)
+#if defined(mingw32_HOST_OS)
+import GHC.IO.Encoding.CodePage (mkLocaleEncoding)
+import GHC.IO.Encoding.Failure (CodingFailureMode(TransliterateCodingFailure))
+#endif
 import HscTypes
 import FastString (unpackFS)
+import MonadUtils (liftIO)
 
 -- | Create 'Interface's and a link environment by typechecking the list of
 -- modules using the GHC API and processing the resulting syntax trees.
@@ -68,6 +75,10 @@ processModules
   -> Ghc ([Interface], LinkEnv) -- ^ Resulting list of interfaces and renaming
                                 -- environment
 processModules verbosity modules flags extIfaces = do
+#if defined(mingw32_HOST_OS)
+  -- Avoid internal error: <stderr>: hPutChar: invalid argument (invalid character)' non UTF-8 Windows
+  liftIO $ hSetEncoding stderr $ mkLocaleEncoding TransliterateCodingFailure
+#endif
 
   out verbosity verbose "Creating interfaces..."
   let instIfaceMap =  Map.fromList [ (instMod iface, iface) | ext <- extIfaces