Merge branch 'add-exception-test' of https://github.com/yui-knk/mruby into yui-knk-add-exception-test

Move `Kernel.local_variables` to core.

This fix use UINT16_MAX for NULL symbol tag, that means maximum symbol length
is not UINT16_MAX-1.

Add Range#size

Add test for remove_instance_variable

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