vec_pmsum_be pictures/examples #33

PC requests a picture and/or example for this. This is higher priority, as the concepts aren't obvious unless you're a cryptography nerd.
(This issue was split from issue #20, which originally had vec_permxor and vec_pmsum_be, but while these are similar, they are not directly related, the work is non-trivial, and it'll be easier to do one at-a-time.)

PC requests a picture and/or example for this. This is higher priority, as the concepts aren't obvious unless you're a cryptography nerd. (This issue was split from issue #20, which originally had `vec_permxor` and `vec_pmsum_be`, but while these are similar, they are not directly related, the work is non-trivial, and it'll be easier to do one at-a-time.)

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vec_pmsum_be pictures/examples #33