Workaround a closure based generator leak in Swift

[onevcat]

This p-r fixes an issue in Swift Generator. This is reported here: #280

After some research, I found when using a closure based generator, a leak will happen in Release build in _GeneratorBox.__allocating_init(A) -> _GeneratorBox. It does not relate to Kingfisher's code, since there are some minimal examples are also reported to be leaking by Instruments.

Turn to a normal GeneratorType instead of closure based one could solve this problem.

[codecov-io]

Current coverage is 79.27%

Merging #281 into master will decrease coverage by -0.07% as of f93eb78

@@            master    #281   diff @@
======================================
  Files           12      12       
  Stmts         1375    1370     -5
  Branches         0       0       
  Methods          0       0       
======================================
- Hit           1091    1086     -5
  Partial          0       0       
  Missed         284     284       

Review entire Coverage Diff as of f93eb78

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