Fix: constant folding on lhs of assignments

[iamdefinitelyahuman]

What I did

Fix a folding issue where constants used in binaries operations inside a subscript were not being correctly folded.

For example, the following currently fails to compile because FOO+1 is not folded:

FOO: constant(int128) = 3
bar: int128[100]

def foo():
    self.bar[FOO+1] = 1

How I did it

The existing logic does not fold constants on the left-hand side of an assignment, unless the constant's parent is a Subscript. To fix this, I'm now checking that the constant has an ancestor who is a Subscript.

How to verify it

Run tests. I expanded the test cases around this functionality to check this specific occurrence.

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[codecov-commenter]

Codecov Report

Merging #2137 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2137   +/-   ##
=======================================
  Coverage   84.90%   84.90%           
=======================================
  Files          83       83           
  Lines        8333     8333           
  Branches     2012     2012           
=======================================
  Hits         7075     7075           
  Misses        755      755           
  Partials      503      503           

Impacted Files	Coverage Δ
vyper/ast/folding.py	91.48% <100.00%> (ø)

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