Fix unsafe usage of arithmetic ops (SlowMist N2)

[clostao]

Description

Slowmist problem description:

In Rust, numeric variables used in calculations without proper overflow checks, such as checked_add ,
checked_mul , or checked_sub , may be susceptible to integer overflow. Integer overflow occurs when the result
of an arithmetic operation exceeds the maximum value that the data type can represent, leading to an unexpected
and potentially unsafe outcome.

Our response:

Suggested changes partially applied.

For some of the required interfaces (e.g pallet-custom-balances) there is no possibility for throwing an error other than panicking. This mechanism is unwanted because could lead to unexpected behaviour so we decided to decided to interpret which case is better in each case to do in case of overflow. For most of the cases, the best solution has been to use Saturating trait and lose some precision in those cases.

This loose of precision are not real potential risks since in most cases these saturating ops. are referred to block numbers or nonces. Having a 32-byte long number make our blockchain invulnerable for trillions of years.

Types of changes

What types of changes does your code introduce?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

[GabrielMartinezRodriguez]

LGTM