Define and implement hard diff removal hardfork

[NyanCatTW1]

Facts of the hard fork:

• Start date: 2025-04-01
• Deadline: 2026-04-01
• Threshold: 95% miner agreement over the time window of 672 blocks (same as deadpool hardfork)
• Activation: When >= 95% of blocks in epoch X support the hardfork, hard diff removal becomes active while determining the difficulty for epoch X+2. There is an epoch where hard diff is still possible (epoch X+1), because it's BIP9's LOCKED_IN retarget period

Concept of the new difficulty adjustment algorithm:

• If block time is too short, increase diff by 1 until we reach an even diff (in do {} while (); manner)
• If block time it too long, decrease diff by 1 until we reach an even diff (in do {} while (); manner)
• If block time is just right and we're at an even diff: Don't change diff
• If block time is just right but we're at an odd diff: Decrease diff by 1. (Why decrease and not increase: My rationale is that the difficulty is currently too high anyways)

What's considered too short/long are identical to the original algorithm (<27 mins is too short, >31 mins is too long)

Introduce more aggressive difficulty adjustment when block time deviates far away (>2x) from 30 minutes:

• If block time is way too short (<15 mins), increase diff by 4 or 3, instead of only by 2 or 1
• If block time is way too long (>60 mins), decrease diff by 4 or 3, instead of only by 2 or 1

Whether it's even or odd depends on whether the original difficulty were even or odd

[ReverseControl]

CalculateDifficultyDelta(): Most of those if statements got to go. We will use bit-operators and ternary operators. Almost every if statement in there is unnecessary.

[ReverseControl]

Approving for now as this has been tested. The optimizations will come in later.