lib/string: StringCharView is no more comparable

Comparing text has only a clear meaning on a text, not on a sequence of
char.

Basically, I think that StringCharView should be private because

* it simplifies the API ; and one wants to aim a simple API for the Text class
* there should be no real services that a StringCharView can do but a
  Sequence[Char] cannot; thus no real gain to type things with the former

Signed-off-by: Jean Privat <jean@pryen.org>
lib/string: teach clients to use non deprecated services.

Mostly, this mean using `chars`.

Signed-off-by: Jean Privat <jean@pryen.org>
lib/string: fast implem for reverse

Signed-off-by: Jean Privat <jean@pryen.org>