When using Scalar::from_bits to manually create unreduced Scalars (e.g.,
X/Ed25519 keys with specified bit patterns), it's possible to construct Scalar
values that range up to 2^255-1. These shouldn't ever end up in a vartime
multiscalar mul call anyways, because it doesn't handle secret data, but it is
technically allowed by the type system and should be handled. When w=8, these
can generate terminal carries that can't be folded into the last digit, but
this can be handled by folding them into an extra digit instead.
This fixes a bug in the Pippenger implementation reported by Fernando Krell and
diagnosed by Oleg Andreev. The problem is that at the largest problem sizes
(using w=8), the signed digits fill the value range of an i8, and so doing
computation on them to calculate the bucket index can hit an overflow.
This was not caught in CI because the test suite didn't check all problem
sizes; tests for these sizes which expose this bug were added in the previous
commit.
