Mark is_ascii_string() as requiring looking at return value

There's no reason to call it otherwise; there are no side effects.
Improve API pod of is_ascii_string
Make is_invariant_string()

This is a more accurately named synonym for is_ascii_string(), which is
retained.  The old name is misleading to someone programming for
non-ASCII platforms.
Bump ext/POSIX version to 1.48
Change core to use is_invariant_string()

is_ascii_string's name has misled me in the past; the new name is
clearer.