dtoa.c: Use an on-stack buffer for small bigints #13234
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Truth is, I don't know how large this is
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In #12991 I replaced a spinlock with malloc, which is about the same performance.
I think malloc implementations have gotten better since 1990 when dtoa.c was written, but this can still be a bit of a bottleneck. We seem to mostly be allocating a few small bigints (k <= 2) at a time, so this PR adds a small on-stack buffer we can allocate from without synchronization.
I need to test this a bit more before merging.