Check length of return data from external calls

[iamdefinitelyahuman]

What I did

• Check the length of return data when making an external call. Closes Vyper doesn't check RETURNDATASIZE on external calls #1978
• Validate that returned bytes and string data does now have a length longer than the max allowed for the given type. Closes No length check on bytes and string arrays #1840

How I did it

• Add a check that RETURNDATASIZE > expected length -1 after making a call.
• In cases where we are expecting return data, do not include the check involving EXTCODESIZE. An address that is not a contract will not return any data, so this check isn't required.
• For dynamic types (bytes and string), assert that the length of the returned data does not exceed the max allowable length for the contract type

It is still possible for a call to return more data than expected by the interface. This is consistent with how Solidity handles the check.

How to verify it

Run the tests. I added a few test cases, and removed one that is no longer valid.

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[codecov-commenter]

Codecov Report

Merging #2076 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2076   +/-   ##
=======================================
  Coverage   85.04%   85.05%           
=======================================
  Files          83       83           
  Lines        8239     8243    +4     
  Branches     1985     1987    +2     
=======================================
+ Hits         7007     7011    +4     
  Misses        735      735           
  Partials      497      497           

Impacted Files	Coverage Δ
vyper/parser/external_call.py	81.69% <100.00%> (+1.09%)	⬆️

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