Integrate checked arithmetics with equal precedence

[philipp2100]

You wrote recently that you don't want checked_arithmetic by default, since this is in the first instance a library for type safety. I understand that, and probably checked arithmetic on unsigned types might also introduce errors.
However, since safe arithmetics are a closely related and important issue, I think they should be available with equal precedence, e.g. to make use of the literal operators to get types with checked arithmetics. This should be a simple change, like creating a new macro TYPE_SAFE_ARITHMETIC_CHECKED.
If you want, I can make a PR.

[foonathan]

Fair enough, I'll merge a PR that modifies the type aliases (but not the template defaults).

[philipp2100]

Great! :)