"Can't unify" and "Can't convert" are terrible error messages for
programmers, and particularly confusing to explain to beginners, because
programmers don't need to know that implementation detail.
Changed to "type mismatch" and updated docs accordingly.
They can be imported from the modules Data.Fin, Data.Vect, and Data.So
respectively.
The general thinking here is that not every program is going to need
these, and they are often used especially by newcomers in place of
something more appropriate. Also, all of them are useful for teaching,
which means it is instructive for tutorials to introduce them and have
people implement them themselves.
This just renames the internal name to something distinguishable. It's
still not completely clear (since it's a renaming from the user's name)
but given that the message means a user has picked the same name twice,
there's not too much else we can do easily... Certainly it's better than
"Can't unify f x with f x"...
Fixes#705
