Remove unnecessary SafeMath calls

Its usage has become ubiquitous, but there are multiple instances where it is not needed. E.g. in ERC721:

function _mint(address to, uint256 tokenId) internal {
  ..
  _ownedTokensCount[to] = _ownedTokensCount[to].add(1);
  ..
}

That is the only place where _ownedTokensCount[to] is incremented: 2^256 calls to _mint are needed to make it overflow, making these calls more expensive by adding the check makes no sense.

[frangio]

Hm, we’ve usually been more of the “use safemath everywhere to be sure” camp. I think we should tackle this by using something like the Counter type that we have, although it doesn’t have a decrement function that is needed for transfers and burning. Thoughts?

[nventuro]

We haven’t yet found a use case for Counter though, have we? If it had methods to both increment and decrement by one, we could remove the require from inc, since it will never overflow. If there is indeed something that only ever increments, then we should create this new thing (IncDec).