GROOVY-11486: rework flatten type parameters, similar to collectMany

[eric-milles]

I tried to follow collectMany in terms of generics. There is no connection that can be made between the in type and the out type, since aggregates are recursively processed. And the closure accepting forms have weaker guarantees for the parameter type, so I added @ClosureParams just to indicate the number of parameters.

https://issues.apache.org/jira/browse/GROOVY-11486

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.

Project coverage is 68.8515%. Comparing base (4baefde) to head (a907045).

Files with missing lines	Patch %	Lines
.../codehaus/groovy/runtime/DefaultGroovyMethods.java	76.9231%	3 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2157        +/-   ##
==================================================
+ Coverage     68.8480%   68.8515%   +0.0035%     
- Complexity      29484      29488         +4     
==================================================
  Files            1421       1421                
  Lines          113264     113264                
  Branches        19584      19584                
==================================================
+ Hits            77980      77984         +4     
+ Misses          28736      28735         -1     
+ Partials         6548       6545         -3     

Files with missing lines	Coverage Δ
.../codehaus/groovy/runtime/DefaultGroovyMethods.java	73.3223% <76.9231%> (ø)

... and 3 files with indirect coverage changes

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