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It turns out the computation we did in the non-negative case alsoworks for the negative case. The proof relies on elementaryproperties of the two's complement representation.I don't know how I never saw this before. To make things worse, itseems that Kotlin and J2CL knew all along, and I never realizedwhen skimming through their implementations either.

Since conversions of signed longs to doubles is in fact no moreexpensive than the unsigned longs, we can take shorter paths forvalues that fit in the *signed* safe range.This applies to the conversions to string (including in thejavalib) and to float.It could also apply to signed division and remainder. However,benchmarks suggest that doing so makes it slower. The trouble isthat we then need a signed double-to-long conversion for theresult, and that appears to be slower than performing the 3 signadjustments.