Add arithmetic overflow detector and improve unprotected initializer detector

[olegggatttor]

The new detector sees if there are assignments/returns that calculate some arithmetic expressions and if some intermediate calculations
contain a type that is lower than the expected result. Such behavior may lead to unexpected overflow/underflow, e.g., trying to assign the multiplication of two uint48 variables to uint256 would look like uint48 * uint48, and it may overflow (however, the final type would fit such multiplication).

[ndkirillov]

Appr!