Add safe_sum and use it in state_processing (#1620)

## Issue Addressed

Closes #1098

## Proposed Changes

Add a `SafeArithIter` trait with a `safe_sum` method, and use it in `state_processing`. This seems to be the only place in `consensus` where it is relevant -- i.e. where we were using `sum` and the integer_arith lint is enabled.

## Additional Info

This PR doesn't include any Clippy linting to prevent `sum` from being called. It seems there is no existing Clippy lint that suits our purpose, but I'm going to look into that and maybe schedule writing one as a lower-priority task.

This theoretically _is_ a consensus breaking change, but it shouldn't impact Medalla (or any other testnet) because `slashings` shouldn't overflow!

consensus/state_processing/src/per_epoch_processing/process_slashings.rs

@@ -1,4 +1,4 @@
-use safe_arith::SafeArith;
+use safe_arith::{SafeArith, SafeArithIter};
 use types::{BeaconStateError as Error, *};
 
 /// Process slashings.
@@ -10,7 +10,7 @@ pub fn process_slashings<T: EthSpec>(
     spec: &ChainSpec,
 ) -> Result<(), Error> {
     let epoch = state.current_epoch();
-    let sum_slashings = state.get_all_slashings().iter().sum::<u64>();
+    let sum_slashings = state.get_all_slashings().iter().copied().safe_sum()?;
 
     for (index, validator) in state.validators.iter().enumerate() {
         if validator.slashed