Explicitly disallow iteration of list of structs

[iamdefinitelyahuman]

What I did

Explicitly disallow iteration of a list of structs

Fixes #2223

How I did it

Add a check within the type checking pass. This behaviour is disallowed in parser, but without the exception in type checking it raises a CompilerPanic

How to verify it

Run the tests. I added a new test to verify this behavior.

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[codecov-io]

Codecov Report

Merging #2228 (19471fb) into master (3c5a809) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2228   +/-   ##
=======================================
  Coverage   85.35%   85.36%           
=======================================
  Files          84       84           
  Lines        8536     8539    +3     
  Branches     2062     2063    +1     
=======================================
+ Hits         7286     7289    +3     
  Misses        744      744           
  Partials      506      506           

Impacted Files	Coverage Δ
vyper/context/validation/local.py	90.20% <100.00%> (+0.12%)	⬆️

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