make private method enumerator_block_call
modify core method that return Enumerator object if not block given
fix return value for with_index
fix self modifying bug
test split and to detail
fix variable name typo
optimize from each to enumerator_block_call
add test for Array#each_index if non block given
Merge pull request #1859 from ksss/enumerator

Fix Enumerator
type of mrb_type() should be enum mrb_vtype when MRB_NAN_BOXING is on
Hash#shift may return any entries
Add Fiber's double resume test and fix it.
symbol length type to be mrb_int
string length type to be mrb_int
calc hash value directly for strings, symbols and numbers
Merge pull request #1863 from take-cheeze/fiber_double_resume

Add Fiber's double resume test and fix it.
Merge branch 'master' of github.com:mruby/mruby