Improve acl

[cbaker6]

• Minimize calls to current user
• setDefaultACL will now throw an error if you attempt to use it with no current user logged in
• Improve docs to reflect actual use case

[codecov]

Codecov Report

Merging #80 (86ace5b) into main (0d81cd4) will decrease coverage by 0.21%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
- Coverage   79.13%   78.92%   -0.22%     
==========================================
  Files          63       63              
  Lines        5018     5020       +2     
==========================================
- Hits         3971     3962       -9     
- Misses       1047     1058      +11     

Impacted Files	Coverage Δ
Sources/ParseSwift/Types/ParseACL.swift	86.09% <81.25%> (-5.86%)	⬇️
Sources/ParseSwift/Objects/ParseObject.swift	75.94% <0.00%> (-0.29%)	⬇️
Sources/ParseSwift/Objects/ParseUser.swift	75.97% <0.00%> (-0.18%)	⬇️

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