Fix decoding of ASCII strings with embedded CR characters

Fix decoding of ASCII strings with embedded CR characters

* src/coding.c (string_ascii_p): Return a negative value if an
all-ASCII string STR includes the CR character, otherwise a
positive value.
(code_convert_string): If the string is ASCII, but includes CR
characters, use the fast path only if EOL doesn't need to be
decoded.  (Bug#40519)

* test/src/coding-tests.el (coding-nocopy-ascii): Add more tests
for bug#40519.