Add the new [Arrow Glacier fork](https://eips.ethereum.org/EIPS/eip-4345).
Only the difficulty calculation is changed, but as a new fork it still affects
a number of places in the code.
To the best of my knowledge the change is only scheduled on Mainnet.
In addition:
- The fork date comments in `chain_config.nim` have been checked against the
real networks, set consistently in UTC instead of random timezones, and made
neater. Maybe we'll keep these when transferring config to a file someday.
- It's added to forkid hash tests (EIP-2124/EIP-2364), of course.
Signed-off-by: Jamie Lokier <jamie@shareable.org>
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