Fix addition overflow in AtomicPosition

[arxanas]

Closes #405.

[arxanas]

Sorry for the delay. Resolved by doing arithmetic in u128.

FWIW, I would advise avoiding the use of as altogether, for the reasons at https://gist.github.com/SimonSapin/550ada9ebedcd21c41cf4e95891e03d7#not-yet-deprecating-the-as-keyword

[djc]

That discussion is pretty old at this point, though. Here's a more recent one:

https://internals.rust-lang.org/t/lets-deprecate-as-for-lossy-numeric-casts/16283/45

Note that this talks about deprecating as for lossy casts, which is not what we're doing here. (There's technically still the risk of the type on the operand changing to something that does result in a lossy cast, but I feel like in this context that risk is not so large.)

[arxanas]

@djc Thanks for merging. Please let me know when there's another release/pre-release so that I can publish a version of my package which depends on indicatif to crates.io.

[djc]

I've published rc10.