* Remove 1 allocation in Enumerable#each_with_index
Previously, each call to Enumerable#each_with_index allocates 2
objects, one for the counting index, the other an imemo_ifunc passed
to `self.each` as a block.
Use `struct vm_ifunc::data` to hold the counting index directly to
remove 1 allocation.
* [DOC] Brief summary for usages of `struct vm_ifunc`