Add ISO index to `Kernel.local_variables`.

Add ISO index to `Kernel.eval` and `Kernel#eval`.

support class define in singleton class

Add a space aftre bracket.

Fix indent in mrb_hash_to_hash()

Use boolean macro in exc_equal.

Remove space.

instance method name use "#" instead of "." for assert

Add comment to Fixnum Class.

Refactor string.c Fix #2267

Use boolean macro in mrb_range_initialize()

Use boolean macro

Delete unnecessary semicolon

Replace tab with spaces.

Fix indents

Add new line behind return type

const_missing error message more detail

Replace tabs with spaces in eval.c

[DOC] fix op code doc

Fix #2259 .