chore: add constant for lean

[syjn99]

What was wrong?

We might need some constants different from the beacon chain. (e.g., INTERVALS_PER_SLOT is 4)

Part of #393.

How was it fixed?

This PR simply add a module in ream-consensus-misc for constant. Also my clippy complains about let-chain so I fixed it also. (Therefore make pr is now happy)

To-Do

• I have read CONTRIBUTING.md.
• This PR title follows conventional commits.

[unnawut]

I'm not sure if ream_consensus_misc needs to host both beacon and lean stuff? They seem entirely independent. Even misc could go into ream_consensus_beacon/lean. But maybe I don't know there's a reason for splitting misc from consensus crate?

But if so I think splitting the crates into one of these pairs make more sense, but just 2 cents:

1. ream_consensus_beacon::misc::constants & ream_consensus_lean::misc::constants
2. ream_misc_beacon::constants & ream_misc_lean::constants but personally 1 makes more sense.

[syjn99]

They seem entirely independent. Even misc could go into ream_consensus_beacon/lean. But maybe I don't know there's a reason for splitting misc from consensus crate?

@unnawut Yeah indeed. I also like the first option, will do that way!

[syjn99]

@unnawut I tried, but removing the existing dependency makes another dependency issues there. As the task size might be too big, I'd say taking this as an issue.

[unnawut]

@unnawut I tried, but removing the existing dependency makes another dependency issues there. As the task size might be too big, I'd say taking this as an issue.

Sgtm. Actually I think this is cosmetic enough we might not even need to tackle at all so an issue is probably not needed.

[jihoonsong]

FYI, the consensus spec is being changed to use milliseconds and basis points: ethereum/consensus-specs#4476