Store carry bits in uint32_t.
Store carry bits in uint32_t.

They're either 0 or 1, so at best this can help compilers realize that
they don't need to do full 128 + 128 or 64 + 64 additions.
At worst, this is a stylistic change to indicate the narrow range.

(This turned out to be unobservable in the benchmark.)

ulfjack/ryu,ulfjack/ryu,ulfjack/ryu,ulfjack/ryu