EIP2929 opcode repricing

[iamdefinitelyahuman]

What I did

• Add EIP2929 opcode repricings
• Set default evm version to berlin

How I did it

I took the new pricings from the EIP2929 spec. I didn't include any logic to account for hot-vs-cold access - in the same way that we always assume an SSTORE to cost 20,000 these values just give an upper-bound estimate.

I also bumped the default evm target to berlin. This has no effect aside from the gas estimates, as there are no new opcodes introduced.

All of this is required to make the test suite pass, because the latest release of eth-tester now uses berlin as the default ruleset.

How to verify it

Verify that the test suite is passing in the CI. I had to tweak a few tests, I tried to do so in a way such that we won't have to adjust them again for the next hardfork.

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[codecov-io]

Codecov Report

Merging #2350 (830cb29) into master (1d2fdf1) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2350      +/-   ##
==========================================
- Coverage   85.88%   85.84%   -0.04%     
==========================================
  Files          90       90              
  Lines        8791     8791              
  Branches     2097     2097              
==========================================
- Hits         7550     7547       -3     
- Misses        753      755       +2     
- Partials      488      489       +1     

Impacted Files	Coverage Δ
vyper/opcodes.py	100.00% <100.00%> (ø)
vyper/functions/functions.py	89.25% <0.00%> (-0.40%)	⬇️

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