Poly1305 panics or computes a wrong tag on attacker-chosen messages of 64 bytes or more

Poly1305 in ic-cipher absorbs messages of 64 bytes or more four blocks at a time, summing four products before one reduction. From 0.2.5 to 0.2.19 the reduction kept its carries in 32-bit integers, which four summed products can overflow when the message's bytes are large. This affects Poly1305 a...