fix an error in go doc
for porfiling the benchmark code, so add flag to disable the test code
rename a var name
Merge branch 'master' of github.com:fanliao/go-plinq into develop
Merge branch 'develop' of github.com:fanliao/go-plinq into develop
update performance testing
try to improve the performance of FirstBy
Merge branch 'develop' of github.com:fanliao/go-plinq into develop
add draft version of ringlist
remove ringList
debug duplicate index issue for Union operation
Add ReIndex field to prevent repeated Order for Union operation
optimize the performance of FirstBy operation:
1. use concurrecy for channel source
2. optimize the preformance around 20% for slice source
optimize the performance of FirstBy operation:
    1. use concurrecy for channel source
    2. optimize the preformance around 20% for slice source