Refactor SafeMath to avoid memory leaks

[Amxx]

FIxes #2413, remove the string allocation in SafeMath default functions: caused unrecovered memory allocation even for successfull calls

[frangio]

After discussing offline we agreed on the following:

• We will deprecate the existing functions like sub(uint, uint, string) because they leak memory. (This is adding a comment in the source code and in the release notes.)
• We will implement sub(uint, uint) by directly calling require so it does not leak memory.
• We will add a new family of functions that return a pair (bool, uint) that can be used to protect against overflow with custom revert reasons, and use these internally in the library. The name for these functions is TBD.

(This also implies that the new add(uint, uint, string) and mul(uint, uint, string) functions added in this PR should be removed.)

[frangio]

We have to add the same deprecation notice for EnumerableMap.get. We use it in ERC721.ownerOf so while we're at it we should probably change that too.

[frangio]

Awesome work thanks!