fix[parser]: block value assignment in for stmt

[charles-cooper]

block statements of the form for x: uint256 = 5 in ...; it should not be allowed.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.06%. Comparing base (2ffc8b3) to head (c720176).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4492   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files         120      120           
  Lines       17330    17337    +7     
  Branches     2932     2936    +4     
=======================================
+ Hits        15955    15962    +7     
  Misses        957      957           
  Partials      418      418           

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