Fix sign warnings for idx.
Fix sign warnings for idx.

indexForExponent() returns uint32_t. This idx is used in only three
expressions: pow10BitsForIndex() and lengthForIndex() take uint32_t,
and `POW10_OFFSET[idx]` doesn't care. Therefore, we can safely
change its type to uint32_t.

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