Enumrable#each_with_index return Enumerator if non block given
 Add Enumerable#sort_by test
Add Enumerable#first test
mark root fiber object; close #1865
add fiber assertion from #1865 patch
add Fiber#== from #1865 patch
Reduce implicit type casts.
rescue StopIteration only.
Merge pull request #1884 from monaka/pr-reduce-implicit-type-cast

Reduce implicit type casts.
Merge pull request #1886 from iij/pr-kernel-loop-rescue-error

Kernel.loop should not catch StandardError.
Merge pull request #1883 from suzukaze/add-test-enumerable

Add enumerable test
Merge pull request #1880 from ksss/enum-each_with_index

Enumrable#each_with_index return Enumerator if non block given
Add Enumerable#count
Add Enumerable#count test
stricter check for double resume; close #1885
Merge pull request #1887 from suzukaze/add-enumerable.count

Add Enumerable#count
should not use nil as default value for Enumerable#count since it prevent counting nil in enums; ref #1887
add a space after C reserved words
change mrb_int to size_t that would take strlen()
string functions arguments changed from mrb_int to size_t that would take strlen()