Better types

[manpages]

I think that it's a non-issue, but still I'd love to give it a thought.

Currently all the types defined to operate in Turtle are just aliases for Text.

I think that instead of tagging those types (which is safer, as it won't get through OverloadedStrings, but cumbersome) we should hide "constructors" and expose smart constructors to the end user.

I'm not even sure that we have to do that, in fact.