Update solidity-parser/parser to 0.20.1

[cgewecke]

Adds custom storage layout support

https://github.com/solidity-parser/parser/releases/tag/v0.20.0

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.62%. Comparing base (e2126e7) to head (554f40d).
Report is 4 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #905   +/-   ##
=======================================
  Coverage   96.62%   96.62%           
=======================================
  Files          19       19           
  Lines        1066     1066           
=======================================
  Hits         1030     1030           
  Misses         36       36           

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[cgewecke]

Zeppelin's failing with this change due to OZ 5577

Error in plugin solidity-coverage: Error: Could not instrument: access/extensions/AccessControlEnumerable.sol. (Please verify solc can compile this file without errors.) mismatched input 'at' expecting {'from', 'error', 'address', 'calldata', 'revert', 'callback', 'leave', 'payable', 'global', 'constructor', 'receive', Identifier} (38:34)

HardhatPluginError: Error: Could not instrument: access/extensions/AccessControlEnumerable.sol. (Please verify solc can compile this file without errors.) mismatched input 'at' expecting {'from', 'error', 'address', 'calldata', 'revert', 'callback', 'leave', 'payable', 'global', 'constructor', 'receive', Identifier} (38:34)
    at SimpleTaskDefinition.action (/home/circleci/project/openzeppelin-contracts/node_modules/solidity-coverage/plugins/hardhat.plugin.js:294:35)