Remove the notion of an "unknown format"
As mentioned in #9456, the format! syntax extension would previously consider an empty format as a 'Unknown' format which could then also get coerced into a different style of format on another argument. This is unusual behavior because `{}` is a very common format and if you have `{0} {0:?}` you wouldn't expect them both to be coereced to the `Poly` formatter. This commit removes this coercion, but still retains the requirement that each argument has exactly one format specified for it (an empty format now counts as well). Perhaps at a later date we can add support for multiple formats of one argument, but this puts us in at least a backwards-compatible situation if we decide to do that.
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