Remove saturating arith from state_processing (#1644)

## Issue Addressed

Resolves #1100

## Proposed Changes

* Implement the `SafeArith` trait for `Slot` and `Epoch`, so that methods like `safe_add` become available.
* Tweak the `SafeArith` trait to allow a different `Rhs` type (analagous to `std::ops::Add`, etc).
* Add a `legacy-arith` feature to `types` and `state_processing` that conditionally enables implementations of
  the `std` ops with saturating semantics.
* Check compilation of `types` and `state_processing` _without_ `legacy-arith` on CI,
  thus guaranteeing that they only use the `SafeArith` primitives 🎉

## Additional Info

The `legacy-arith` feature gets turned on by all higher-level crates that depend on `state_processing` or `types`, thus allowing the beacon chain, networking, and other components to continue to rely on the availability of ops like `+`, `-`, `*`, etc.

**This is a consensus-breaking change**, but brings us in line with the spec, and our incompatibilities shouldn't have been reachable with any valid configuration of Eth2 parameters.

consensus/state_processing/src/per_block_processing/verify_exit.rs

@@ -3,6 +3,7 @@ use crate::per_block_processing::{
     signature_sets::{exit_signature_set, get_pubkey_from_state},
     VerifySignatures,
 };
+use safe_arith::SafeArith;
 use types::*;
 
 type Result<T> = std::result::Result<T, BlockOperationError<ExitInvalid>>;
@@ -77,11 +78,14 @@ fn verify_exit_parametric<T: EthSpec>(
     );
 
     // Verify the validator has been active long enough.
+    let earliest_exit_epoch = validator
+        .activation_epoch
+        .safe_add(spec.shard_committee_period)?;
     verify!(
-        state.current_epoch() >= validator.activation_epoch + spec.shard_committee_period,
+        state.current_epoch() >= earliest_exit_epoch,
         ExitInvalid::TooYoungToExit {
             current_epoch: state.current_epoch(),
-            earliest_exit_epoch: validator.activation_epoch + spec.shard_committee_period,
+            earliest_exit_epoch,
         }
     );