Add API `mrb_atexit()`.
`mrb_final_mrbgems` will be called as mrb_state atexit function.
Maybe useful in #1844.
Fix #2259 .
include/mruby/opcode.h fix operation doc
src/vm.c fix operation doc
[ci skip]
const_missing error message more detail
Replace tabs with spaces in eval.c
Add new line behind return type
Merge pull request #2261 from take-cheeze/fix_2259

Fix #2259 .
Merge pull request #2262 from ksss/doc-opcode

[DOC] fix op code doc
Merge pull request #2263 from suzukaze/replace-tab-with-space

Replace tabs with spaces in eval.c
Merge pull request #2264 from ksss/const_missing

const_missing error message more detail
Merge pull request #2265 from suzukaze/refactor

Add new line behind return type
Fix indents
Merge pull request #2266 from suzukaze/fix-indent

Fix indents
Replace tab with spaces.
Delete unnecessary semicolon
Use boolean macro
Use boolean macro in mrb_range_initialize()
Merge pull request #2268 from yui-knk/replace-tab

Replace tab with spaces.
Merge pull request #2269 from suzukaze/delete-semicolon

Delete unnecessary semicolon
Merge pull request #2270 from suzukaze/refactor-class.c

Use boolean macro