fix bug in pop! and popfirst!

[sl-solution]

This changes invpool to pool in pop! and popfirst!.

[codecov]

Codecov Report

Merging #77 (8fc66c4) into main (53f57d4) will increase coverage by 2.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #77      +/-   ##
==========================================
+ Coverage   87.00%   89.12%   +2.11%     
==========================================
  Files           1        1              
  Lines         331      331              
==========================================
+ Hits          288      295       +7     
+ Misses         43       36       -7     

Impacted Files	Coverage Δ
src/PooledArrays.jl	89.12% <100.00%> (+2.11%)	⬆️

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