Removesafe_ prefixes from math functions

[aritkulova]

• This PR suggests a bug fix and I've added the necessary tests.
• This PR introduces a new feature and I've discussed the update in an Issue or with the team.
• This PR is just a minor change like a typo fix.

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This PR is a follow-up to #57 and makes function naming consistent:

• removes the _safe prefix so math functions are safe by default;
• renames add -> add_carry;
• renames sub -> sub_borrow;
• renames mul -> mul_wide.

[Hrom131]

@aritkulova

What do you think about removing the safe_ prefix from all the remaining math operations (add, mul, sub)? It seems a little odd that they all still have the prefix, while div doesn't. I think we should keep the checked_ prefix and leave the functions without a prefix, which are safe by default.

[aritkulova]

@Hrom131
Although add, sub, and mul still make sense as operations that can potentially overflow, I agree that removing the safe_ prefix would be more consistent.
I believe we should keep the current add, sub, and mul for u128 and u256 as unsafe_, since they are still useful. Should we return unsafe_div (for u128 and u256) for consistency then?

[Hrom131]

LGTM!