Expand checked arithmetic to more crates

[michaelsproul]

Description

In #1009 we added lints for unchecked integer arithmetic and switched to checked arithmetic in the types and state_processing crates. This issue is about expanding that coverage to more crates, probably in eth2/utils to start with (i.e. the direct dependencies of state_processing).

[michaelsproul]

Another one to remove, .abs() from fork choice