add Latin1 encoder for use with HTTP headers

Presently there is a function `Data.Text.Encoding.decodeLatin1` which decodes a ByteString as Latin1/ISO-8859-1 text to `Text`.
It would be very helpful to have a corresponding `encodeLatin1` function to assist in writing HTTP header serialization in i.e. Servant.

This would obviously not be a total function as most of Unicode cannot be coded by Latin1, however, it could be made to either throw exceptions or return an `Either` value and still be very helpful.