Should be MRB_DATA class. RStruct isn't managed by mruby core. So it should be self-managed class instances.

Remove unused struct and more

format some source code (class.c)

Use typedef names as possible

Fix arena size check.

Adjust space

Move Math::TOLERANCE to mruby-math

Modification of CONTRIBUTING.md

Some minor fixes of the  CONTRIBUTING.md file.

s/mgem/mrbgem/

Do not generate test_args_hash variable if it is not used

Cleanup Regexp codes

Rake respects verbose flag also for test and clean target

Regexp: %r notation bug fix.

Pluggable Math

be able to parse any delimiters.
variable expansion isn't supported yet.

 * sample code
```
def matchpath(str)
  puts "#{str} is ..."

  if %r!img/! =~ str then
    puts "match"
  else
    puts "not match"
  end
end

matchpath("img/a.png") # => match
matchpath("/usr/local/bin") # => not match
matchpath("book/img/local") # => match
```