Reverse fingerprint order

[georgwiese]

This PR changes the fingerprint of a tuple $(x_1, ..., x_n)$:
from
$\sum_{i=1}^n \alpha^{(n - i)} x_i$
to
$\sum_{i=1}^n \alpha^{(i - 1)} x_i$

The difference is that previously, you could pad any number of zeros to the left and get the same fingerprint. Now, you can pad any number of zeros to the right, which I find more intuitive.

This also fixes a potential soundness bug: In practice, we always use fingerprint_with_id, which prepends the bus ID to the payload, e.g. (<BUS_ID_FOO>, a, b, c) & (<BUS_ID_BAR>, a, b). But fingerprint((<BUS_ID_BAR>, a, b)) == fingerprint((0, <BUS_ID_BAR>, a, b)), so if BUS_ID_FOO is 0, the receiver could be ambiguous.

This is fixed now, because every bus interaction will at least fingerprint the bus ID, which is always the first element.