Optimized groupBy operations

[l0rinc]

Fixes #1345

Before:

Target         Operation                               Ratio          10        100       1000 
ListBenchmark  GroupBy     fjava_persistent/slang_persistent       0.90x      0.13x      0.05x 
ListBenchmark  GroupBy         java_mutable/slang_persistent      19.23x     20.13x     15.16x 
ListBenchmark  GroupBy     scala_persistent/slang_persistent       4.96x      8.49x     10.34x 

i.e. it was ~10x slower than Scala

After:

Target         Operation                               Ratio          10        100       1000 
ListBenchmark  GroupBy     fjava_persistent/slang_persistent       0.11x      0.01x      0.00x 
ListBenchmark  GroupBy         java_mutable/slang_persistent       2.55x      2.00x      1.19x 
ListBenchmark  GroupBy     scala_persistent/slang_persistent       0.66x      0.83x      1.04x

i.e. it's usually faster than Scala

[l0rinc]

wouldn't compile otherwise

[codecov-io]

Current coverage is 95.57%

Merging #1350 into master will increase coverage by <.01%

1. 2 files (not in diff) in .../main/java/javaslang were modified. more
   • Partials -2
   • Hits +2

@@             master      #1350   diff @@
==========================================
  Files            86         86          
  Lines          9988       9991     +3   
  Methods           0          0          
  Messages          0          0          
  Branches       1802       1803     +1   
==========================================
+ Hits           9543       9548     +5   
  Misses          318        318          
+ Partials        127        125     -2   

Powered by Codecov. Last updated by 930163f...0f1bf89

[danieldietrich]

Great, thank you!