These two static members are now used to implement respective `matches_`
methods but will also be useful to provide a global implementation of
the specified concept of whitespace.
This was previously a slightly confusing API. Even when there was no EOL
marker at the current location, we would still consume one byte.
It will now consume either EOL or nothing at all.
The way this was set up before, this function would return "true" if
the underlying stream had ended, which would cause us to try to read
past the end in some edge cases.
