Add ISO index to `Kernel.local_variables`.
Add ISO index to `Kernel.eval` and `Kernel#eval`.
Add test for remove_instance_variable
Merge pull request #2290 from take-cheeze/eval_iso

Add ISO index to `Kernel.eval` and `Kernel#eval`.
Merge pull request #2289 from take-cheeze/local_variables_iso

Add ISO index to `Kernel.local_variables`.
Merge pull request #2291 from carsonmcdonald/removeivtest

Add test for remove_instance_variable
Add Range#size
Merge pull request #2293 from suzukaze/range.size

Add Range#size
reimplement Range#size using double with fixing corner cases; ref #2293
allow NULL (no variable) in lvar section of mrb format; fix #2294

This fix use UINT16_MAX for NULL symbol tag, that means maximum symbol length
is not UINT16_MAX-1.
UINT16_MAX as symbol length is reserved; ref #2294