Fixing the handling of msg.signer in contracts

[fjarri]

... and all the related issues that popped up:

• added AccountSigner.private_key property
• added LocalProvider.add_account() method
• added an optional sender_address parameter of ClientSession.eth_call()
• AccountSigner now takes LocalSigner specifically and not just any BaseSigner
• ClientSession.estimate_transact() and estimate_deploy() now require a sender_address

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (85a956c) 100.00% compared to head (1324e2d) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #62   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         2018      2024    +6     
  Branches       335       336    +1     
=========================================
+ Hits          2018      2024    +6     

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