crypto(fixpoint): add overflow failure test

[erwanor]

This test should fail and overflow loudly, but instead it fails silently producing a nonsensical value.

[hdevalence]

The bug is in this code I wrote in U128x128::checked_mul:

        x1y1.checked_shl(128)
            .and_then(|acc| acc.checked_add(x0y1))
            .and_then(|acc| acc.checked_add(x1y0))
            .and_then(|acc| acc.checked_add(x0y0 >> 128))
            .map(U128x128)

The first call was intended to return None when there are nonzero bits in the high part of the product. But that's not what checked_shl does: the check is on the shift amount, not the shifted bits. Oops.

To remediate this, we should:

• Use U256::into_words to split the high and low parts of x1y1, and check that the high part is zero, erroring out if so
• Add a fixpoint::Error enum with an Error::Overflow variant using thiserror, and swap out the use of Options in the U128x128 API for Results
• Make a plan of how to review use of the U128x128 arithmetic in the DEX code to check that it either doesn't overflow or errors out semi-gracefully

[hdevalence]

(The last point can be a followup issue)