The Text variant of Document now contains a Cow<str> instead of a String. This
small modification triggered a lot of lifetime and type errors accross the
codebase.

This commits fixes these errors, in the most conservative way possible. Tests
have been fixed too, so that they still compile (and pass).

Next commits will focus on completely removing the usage of String in Document.

This commit removes every usage of String in Document, replacing it with the
Cow<str> type.

The work is not finished yet! A lot of clone still have to be removed. This will
be the goal of the next few commits.

Tests have been updated, and still pass.

The call to clone was removed in the implementation of Documentable for &str.

Warning: the code currently does not compile. The compilation errors are due to
pattern-matching. These errors *should* be fixed with the `butcher` crate,
which *should* significantly reduce the amount of duplication generated by
pattern-matching on enums wrapped in Cow.

Butcher will make Cow handling a bit less painfull, and will remove a lot of code
duplication.

This conflict comes from commit d4e11ea.