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/safe_arith/src/lib.rs

@@ -1,4 +1,7 @@
 //! Library for safe arithmetic on integers, avoiding overflow and division by zero.
+mod iter;
+
+pub use iter::SafeArithIter;
 
 /// Error representing the failure of an arithmetic operation.
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
@@ -7,7 +10,7 @@ pub enum ArithError {
     DivisionByZero,
 }
 
-type Result<T> = std::result::Result<T, ArithError>;
+pub type Result<T> = std::result::Result<T, ArithError>;
 
 macro_rules! assign_method {
     ($name:ident, $op:ident, $doc_op:expr) => {