lib/string: StringCharView is no more comparable
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 <51f8b1fa9b424745378826727452997ee2a7c3d7@pryen.org>

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