Rename "Merge" -> "Bellatrix"

[hwwhww]

Merge -> Bellatrix

Description

As per ethereum/pm#430 discussion, we will use Bellatrix as the code name for the next CL hard fork.

This PR:

1. Move the files from */merge/ to */bellatrix/ folder
2. Replace the keyword "Merge" with "Bellatrix" in most places
3. Update the names of fork-specific configuration and preset items

I kept the word "merge" when it is about the point of "merge transition".

Configuration name changes

Old name	New name
MERGE_FORK_VERSION	BELLATRIX_FORK_VERSION
MERGE_FORK_EPOCH	BELLATRIX_FORK_EPOCH
GOSSIP_MAX_SIZE_MERGE	GOSSIP_MAX_SIZE_BELLATRIX
MAX_CHUNK_SIZE_MERGE	MAX_CHUNK_SIZE_BELLATRIX

Preset name changes

Old name	New name
INACTIVITY_PENALTY_QUOTIENT_MERGE	INACTIVITY_PENALTY_QUOTIENT_BELLATRIX
MIN_SLASHING_PENALTY_QUOTIENT_MERGE	MIN_SLASHING_PENALTY_QUOTIENT_BELLATRIX
PROPORTIONAL_SLASHING_MULTIPLIER_MERGE	PROPORTIONAL_SLASHING_MULTIPLIER_BELLATRIX

TODO

• Check if test generators work well

[mkalinin]

Congratulations on the new name! 🥳

I have commented a few places where the Merge is still used in the context of the fork name, another place is /phase0/deposit-contract.md:

• "in a followup fork after the Merge"
• "until sometime after the Merge"

Other than that, looks good to me! Let's get it merged after a few fixes!

[ralexstokes]

phew nice job!!

i had two minor questions below, but otherwise looks good!