cf9733d678 Verify and fix mul_u64 (Tobin C. Harding)
Pull request description:
Add kani verification for `U256::mul_u64`, doing so uncovered a bug in the current implementation due to overflow. Re-write the `mul_u64` method.
Fix: #1497
## Note
This PR now _only_ tests `mul_u64`, I will add testing div as a separate PR.
ACKs for top commit:
Kixunil:
ACK cf9733d678
apoelstra:
ACK cf9733d678
Tree-SHA512: c044fb385073bb068412195c079f26057ba560b2a7eca22693c360d324a7c0119cf10da2e70e544c69e05ca8269a81156418d307f4e11d8a4c1c1a7cafa32d0c