More systematic way to identify overflow

[rsheeter]

Due to Chrome enabling overflow checks (https://doc.rust-lang.org/book/ch03-02-data-types.html#integer-overflow) we end up fighting overflow issues, most recently #1286.

We don't want to check the entire codebase for potential overflow, we want a targeted check. The overflow lint is apparently ill suited for the task because we cannot tell it that e.g. Fixed doesn't overflow.

So ... what can we do?

One idea might be to do a targeted scan of MIR? - https://blog.rust-lang.org/2016/04/19/MIR.html

Other ideas?