PR6695: Make sure the compiler only uses ASCII string functions.

This should cover all places involving filenames in the compiler.
There are a few more paths still using Latin-1 in other ways,
e.g. in ocamldoc.
PR6695: Make Filename use only US-ASCII functions.

The only place that includes changes is the code for checking
the suffix. It is highly unlikely that the change has any
impact at all.
PR6694: Deprecate Latin-1 string manipulation functions.

Also, add documentation for the US-ASCII variants.
PR6694: Un-warn-error deprecation warnings in Str to preserve legacy behavior.
Update Changes.