## Proposed Changes
Speed up epoch processing by around 10% by inlining methods from the `safe_arith` crate.
The Rust standard library uses `#[inline]` for the `checked_` functions that we're wrapping, so it makes sense for us to inline them too.
## Additional Info
I conducted a brief statistical test on the block at slot [3858336](https://beaconcha.in/block/3858336) applied to the state at slot 3858335, which requires an epoch transition. The command used for testing was:
```
lcli transition-blocks --testnet-dir ./common/eth2_network_config/built_in_network_configs/mainnet --no-signature-verification state.ssz block.ssz output.ssz
```
The testing found that inlining reduced the epoch transition time from 398ms to 359ms, a reduction of 9.77%, which was found to be statistically significant with a two-tailed t-test (p < 0.01). Data and intermediate calculations can be found here: https://docs.google.com/spreadsheets/d/1tlf3eFjz3dcXeb9XVOn21953uYpc9RdQapPtcHGH1PY
## Proposed Changes
Lots of lint updates related to `flat_map`, `unwrap_or_else` and string patterns. I did a little more creative refactoring in the op pool, but otherwise followed Clippy's suggestions.
## Additional Info
We need this PR to unblock CI.
## 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.
## 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!
