Reduce GENESIS_SLOT to 2**32

[JustinDrake]

• Address the slot/epoch underflow problem, even for Java/Javascript/Typescript implementers! 🎉
• Squash a bug with get_previous_epoch
• Fix Change if statements in get_crosslink_committes #642
• Address The integer debate #626 (Vitalik, Danny, myself agree that avoiding signed integers is probably best)

[Nashatyrev]

I would vote for 2**20. It still addresses underflows, but is easier to remember and perform decimal calculations in mind.

[JustinDrake]

We will reach 2**20 slots ~70 days after genesis, so - 2**20 cannot be done by zeroing a single bit. On the other hand it will take 800+ years to reach slot 2**32 so there's no possibility of overflowing past the 32nd bit.

[Nashatyrev]

@JustinDrake thanks for clarification!
Sorry, I probably missed the rationale for zeroing a single bit instead of doing - GENESIS_SLOT, can you please recall?

[JustinDrake]

It's friendlier to bit operations. For example, we can cast the slot to uint32 (stripping off the top 32 bits), or equivalently (slot << 32) >> 32, or equivalently bitwise XOR slot ^ 2**32.

Unfortunately there isn't a single canonical way forward so it's liable to bikeshedding. Vitalik independently suggested 2**32 on a call recently 😂 🤷‍♂️I think we all don't care too much.