Inline safe_arith methods (#3229)

## 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

lcli/src/main.rs

@@ -103,7 +103,13 @@ fn main() {
                         .required(true)
                         .default_value("./output.ssz")
                         .help("Path to output a SSZ file."),
-                ),
+                )
+                .arg(
+                    Arg::with_name("no-signature-verification")
+                        .long("no-signature-verification")
+                        .takes_value(false)
+                        .help("Disable signature verification.")
+                )
         )
         .subcommand(
             SubCommand::with_name("pretty-ssz")