feat: introduce block.prevrandao as alias for block.difficulty

[pcaversaccio]

What I did

EIP-4399 renames the DIFFICULTY (0x44) opcode to PREVRANDAO (0x44). The return value of the DIFFICULTY (0x44) instruction after this change is the output of the randomness beacon provided by the beacon chain.

How I did it

With my brain.

How to verify it

Ask Vitalik's masternode.

Commit message

feat: add `block.prevrandao` as alias for `block.difficulty`

per paris merge, EIP-4399 renames the `DIFFICULTY (0x44)` opcode to
`PREVRANDAO (0x44)`. the return value of the `DIFFICULTY (0x44)`
instruction after this change is the output of the randomness beacon
provided by the beacon chain.

Description for the changelog

Add block.prevrandao as alias for block.difficulty.

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #3085 (7423bd1) into master (915d430) will decrease coverage by 0.02%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #3085      +/-   ##
==========================================
- Coverage   88.36%   88.33%   -0.03%     
==========================================
  Files          98       98              
  Lines       11020    11028       +8     
  Branches     2605     2608       +3     
==========================================
+ Hits         9738     9742       +4     
- Misses        832      834       +2     
- Partials      450      452       +2     

Impacted Files	Coverage Δ
vyper/semantics/environment.py	100.00% <ø> (ø)
vyper/codegen/expr.py	84.56% <50.00%> (-0.67%)	⬇️
vyper/evm/opcodes.py	100.00% <100.00%> (ø)

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[pcaversaccio]

@fubuloubu since all conversations are resolved, I think it's time to trigger the CI.

[charles-cooper]

oh i didn't mean to resolve this: https://github.com/vyperlang/vyper/pull/3085/files#r965129658

[charles-cooper]

@pcaversaccio could you add a warning then depending on the EVM version? if the user tries to use block.difficulty in >= paris, warn. if the user tries to use block.prevrandao in pre-paris, warn. the fact that the two usages compile to the same opcode is handy for backwards compatibility reasons, but ultimately we do not want users to be thinking of them as the same thing as they have (very) different semantics.

[pcaversaccio]

@charles-cooper thanks for implementing the warning!

[charles-cooper]

@fubuloubu implemented a warning depending on usage in 7107788. could you please sign off if it makes sense to you? and then i can merge

[charles-cooper]

i resolved my last bit of feedback