Fix length function to measure code units

Fixes #17

Author: hasufell

System/OsString/Common.hs

@@ -502,9 +502,20 @@ null = coerce BSP.null
 
 -- | /O(1)/ The length of a 'OsString'.
 --
+-- This returns the number of code units
+-- (@Word8@ on unix and @Word16@ on windows), not
+-- bytes.
+--
+-- >>> length "abc"
+-- 3
+--
 -- @since 1.4.200.0
 length :: PLATFORM_STRING -> Int
+#ifdef WINDOWS
+length = coerce BSP.numWord16
+#else
 length = coerce BSP.length
+#endif
 
 -- | /O(n)/ 'map' @f xs@ is the OsString obtained by applying @f@ to each
 -- element of @xs@.

tests/bytestring-tests/Properties/Common.hs

@@ -454,6 +454,10 @@ tests =
 
   , ("length" ,
    property $ \x -> numWord x === fromIntegral (length (B.unpack x)))
+#if defined(OSWORD) || defined(WIN) || defined(POSIX)
+  , ("length abc" ,
+   once $ B.length (B.pack [0xbb, 0x03]) == 2)
+#endif
   , ("count" ,
    property $ \(toElem -> c) x -> B.count c x === fromIntegral (length (elemIndices c (B.unpack x))))
   , ("filter" ,