Add unsafe decoding functions

[oberblastmeister]

unsafeDecodeUtf8 :: ByteString -> Text
unsafeDecodeUtf8ByteArray :: ByteArray -> Int -> Int -> Text

These functions do not check if the input is valid utf8. unsafeDecodeUtf8ByteArray does not check the bounds either.

[ˌbodʲɪˈɡrʲim]

There is a maintenance price to expand API with all helpers possible, I'm reluctant to go this route.