feature: add for uint storageguards

[prestwich]

Adds an add implementation to &mut MutStorageGuard to allow for direct addition

Goal is to allow this in contracts

self.balance_mapping.setter(sender_address) += U256::from(10000);

This is untested.

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[qalisander]

I think that SubAssign, MulAssign, DivAssign should be added also.