Add API `mrb_atexit()`.
`mrb_final_mrbgems` will be called as mrb_state atexit function.
Maybe useful in #1844.
fix indent.
Merge pull request #2324 from yui-knk/fix-indent

fix indent.
Fix invalid array initialize for VS2012
Merge pull request #2329 from bggd/patch-4

Fix invalid array initialize for VS2012
Support string key in `Struct#[]=`.
Add invalid key type check in `Struct#[]=`.
Merge pull request #2330 from take-cheeze/struct_set_str

Improve `Struct#[]=`.
add internal function mrb_toplevel_run_keep() to keep stack contents; close #2326
Remove space.
Merge pull request #2334 from yui-knk/remove-space

Remove space.
Fix typos in gc
Merge pull request #2336 from katmutua/master

Fix typos in gc
remove part of 9cd71916 test for same reason as 5306e47
Merge branch 'master' of github.com:mruby/mruby
Remove space.
Fix indent.
Merge pull request #2342 from yui-knk/fix-indent

Fix indent.
Merge pull request #2341 from yui-knk/remove-space

Remove space.
Fix mgem test args passing