add Integer#div and Float#div.
Merge pull request #1791 from iij/pr-numeric-div

add Integer#div and Float#div to mruby-numeric-ext
fix a comment end of include guard
fix off-by-one error in String#rindex(fixnum)

null-terminated string should not be included in search targets.
Merge pull request #1792 from cubicdaiya/issues/fix_invalid_comment

fix a comment end of include guard
Merge pull request #1795 from cubicdaiya/issues/off_by_one_error

fix off-by-one error in String#rindex(fixnum)