Eliminate bunch of copies of error codepath from Utf8LossyChunksIter

Using a macro to stamp out 7 identical copies of the nontrivial slicing
logic to exit this loop didn't seem like a necessary use of a macro. The
early return case can be handled by `break` without practically any
changes to the logic inside the loop.

All this code is from early 2014 (7.5 years old, pre-1.0) so it's
possible there were compiler limitations that forced the macro way at
the time.

Confirmed that `x.py bench library/alloc --stage 0 --test-args from_utf8_lossy`
is unaffected on my machine.

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