This implements a type with a compact representation of `[Char]`.

The data is stored internally as UTF8 in an 'Data.ByteString.Short.ShortByteString'
when compiled against `bytestring >= 0.10.4`, and otherwise in a
plain old `[Char]`.

`ShortByteString` is available only from `bytestring` 0.10.4 on, and GHC
7.8.4 was the first GHC to bundle `binary-0.10.4`. So this fallback
affects mostly only GHC 7.6 and older.

Note: Originally a strict `ByteString` was used as fallback for this patch. However, the 
`[Char]` fallback avoids pinned memory and may be more preferable when dealing with
many small `ShortText`s