Fix ASCII-only conversion logic (bug#40407)

Fix ASCII-only conversion logic (bug#40407)

To sidestep conversion altogether when EOL conversion applies, we must
either be encoding a string without NL, or decoding without CR.

* src/coding.c (string_ascii_p): Revert to a pure predicate.
(code_convert_string): Fix logic.  Don't use uninitialised
ascii_p (removed).  Use memchr to detect CR or LF in string when needed.
* test/src/coding-tests.el (coding-nocopy-ascii):
Update tests to include encodings with explicit EOL conversions.