U8String / U8String / 8798865629

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fix: revert SWAR changes in ASCII case converter - I autopiloted them, writing fast but wrong code yet again
fix: handle length edge cases better in invariant converter
test: add exhaustive coverage for short length and rune permutations for case converters (4 millions of them)
perf: better codegen for .Concat(ROSpan<TFormattable>), add "fast" path for UTF-32->UTF-8 conversion (until simdutf impl. is adopted)
perf: support immutable arrays in TryGetSpan dispatch helper, because currently it can't be guaranteed that they will always be passed to non-boxing overloads due to overload resolution limitations and overload spam

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