Add hardhat to peerDependencies

[kevinji]

This fixes a compatibility issue with Yarn strict pnpMode, as solidity-coverage calls require() on hardhat.

[codecov-commenter]

Codecov Report

Merging #722 (4d11197) into master (bc7d076) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #722   +/-   ##
=======================================
  Coverage   95.90%   95.90%           
=======================================
  Files          19       19           
  Lines        1051     1051           
=======================================
  Hits         1008     1008           
  Misses         43       43           

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[kevinji]

@cgewecke do you mind reviewing this?

[cgewecke]

Thanks, looks good!