Audit code for `U256` overflows

[Bhargavasomu]

This would be done once the code is in good shape in terms with VM operation and chain sync.

[SamWilsn]

Is the goal be to find places that U256 could overflow and either replace with, as appropriate, one of:

• U256.wrapping_add; or
• Conversion to Uint?

[Bhargavasomu]

I opened it mostly focussing on conversion to Uint, but it would also be good to cross check that the wrapping operations are in place wherever appropriate.

[github-actions]

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