Fix unused storage slots

[charles-cooper]

What I did

fix: don't alloc slots for nonreentrant keys which appear multiple times.

this is not a semantic bug but an optimization bug since we allocate
more slots than we actually need, leading to "holes" in the slot
allocator -- slots which are allocated but unused.

How I did it

How to verify it

Description for the changelog

fix: don't allocate slots for nonreentrant keys which appear multiple times.

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[codecov-commenter]

Codecov Report

Merging #2439 (b5b6705) into master (cec2c53) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2439   +/-   ##
=======================================
  Coverage   84.63%   84.63%           
=======================================
  Files          94       94           
  Lines        9273     9276    +3     
  Branches     2093     2094    +1     
=======================================
+ Hits         7848     7851    +3     
  Misses        940      940           
  Partials      485      485           

Impacted Files	Coverage Δ
vyper/semantics/validation/data_positions.py	93.10% <100.00%> (+0.79%)	⬆️

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[fubuloubu]

Can you rebase on master to incorporate #2433?

[charles-cooper]

Sure. I'm not so sure anymore this is such a useful optimization tho. It only helps if there are more than 255 (storage variables + nonreentrant keys)

[charles-cooper]

related: #2436

[charles-cooper]

Alright, along with b5b6705 I feel pretty confident about this PR.