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.

.github/workflows/test-suite.yml

@@ -115,6 +115,14 @@ jobs:
     - uses: actions/checkout@v1
     - name: Typecheck benchmark code without running it
       run: make check-benches
+  check-consensus:
+    name: check-consensus
+    runs-on: ubuntu-latest
+    needs: cargo-fmt
+    steps:
+    - uses: actions/checkout@v1
+    - name: Typecheck consensus code in strict mode
+      run: make check-consensus
   clippy:
     name: clippy
     runs-on: ubuntu-latest