Fix two typos

format some source code (vm.c)

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/