TODO tests for opening upgraded scalars
fail to open scalars containing characters that don't fit in a byte
document the new warning
bump PerlIO::scalar's version
TODO tests for reads from a scalar changed to upgraded after open
handle reading from a SVf_UTF8 scalar

if the scalar can be downgradable, it is downgraded and the read succeeds.

Otherwise the read fails, producing a warning if enabled and setting
errno/$! to EINVAL.
TODO tests for writing to a SVf_UTF8 scalar
warn and fail on writes to SVf_UTF8 SVs
[perl #109828] disallow scalar I/O on non-byte strings