Add Text instance

We should support the Text datatype. I don't think there are any dependency issues with doing so given that text is in the Platform and in any case HTTP already depends indirectly on text via network and parsec 3.

However any instance would have the same problems with encoding as in #28, so it would make sense to fix that first.

Start by reading issue #28 to understand the encoding problems that must be resolved first, then inspect the existing datatype instances in the HTTP package. Done means the Text datatype is supported without those encoding issues and the relevant package behavior is verified.

Written by the indexing model from the issue text.