Actual implementation of IsCharOk() didn't correspond to the comments in
it or near the function declaration which stated that it's never called
with ch='-' as argument -- it was called with it and called IsMinusOk()
right before a comment saying that it doesn't need to do it, which was
very confusing.
Fix this by making the behaviour really correspond to the comments and
handling '-' at the base class level. This required introducing a new
pure virtual CanBeNegative() function, but it's going to be useful for
other things later too.
Still keep IsMinusOk() helper, but make it private now because it
doesn't need to be called from the derived class IsCharOk() any longer.
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