Arrow Glacier spec

[timbeiko]

Adds a spec placeholder for Arrow Glacier, as agreed upon on AllCoreDevs 123 ethereum/pm#391

[codecov-commenter]

Codecov Report

Merging #362 (507e6bc) into master (d414efa) will increase coverage by 1.41%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
+ Coverage   94.07%   95.49%   +1.41%     
==========================================
  Files          13       46      +33     
  Lines         827     2441    +1614     
==========================================
+ Hits          778     2331    +1553     
- Misses         49      110      +61     

Flag	Coverage Δ
unittests	95.49% <ø> (+1.41%)	⬆️

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Impacted Files	Coverage Δ
src/ethereum/base_types.py	89.13% <0.00%> (-4.03%)	⬇️
src/ethereum/vm/__init__.py
src/ethereum/eth_types.py
src/ethereum/trie.py
src/ethereum/vm/instructions.py
src/ethereum/spec.py
src/ethereum/vm/interpreter.py
src/ethereum/vm/error.py
src/ethereum/vm/stack.py
src/ethereum/vm/gas.py
... and 47 more

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