Fix SignersInfo packing issue

[immrsd]

A bug was discovered in the original StorePacking trait impl for SignersInfo: the quorum value was multiplied by an incorrect 2**32 value (0xffffffff instead of 0x100000000). The only case when this incorrect multiplication leads to incorrect unpacking is when signers_count is 0xffffffff (4_294_967_295), causing the quorum to exceed the expected value by one and the signers_count to unpack to 0. If all other use cases the unpacked value equal the one that was packed, since the same constant of 0xffffffff is used for division.

Nevertheless, this issue needs to be addressed.