add macro ARY_SHARED_P
add macro ARY_SET_SHARED
add macro ARY_UNSET_SHARED
use C style comments instead of C++ style comments

According to CONTRIBUTING.md,

Don't use C++ style comments

 /* This is the prefered comment style */

Use C++ style comments only for temporary comment e.g. commenting out some code lines.
remove unnecessary .gitkeep
Merge branch 'array-shared-macro' of https://github.com/ksss/mruby into ksss-array-shared-macro
add _FLAG to SET_SHARED macros; ref #1750
need to initialize flags for pooled strings; close #1768
Merge pull request #1773 from tmash06/remove_unnecessary_gitkeep

remove unnecessary .gitkeep
Merge branch 'issues/comment_style' of https://github.com/cubicdaiya/mruby into cubicdaiya-issues/comment_style
revert changes to mrbconf.h
Merge branch 'master' of github.com:mruby/mruby