Move src/re.h to include/mruby/re.h .
Use mruby/re.h instead of duplicate definition.
convert exception message to string

fixes: mruby -e "p ArgumentError.new(1)"
get classname and concatenate ": " only once
various functions should have internal linkage
Merge pull request #2067 from take-cheeze/move_re_h

Move src/re.h to include/mruby/re.h .
Merge pull request #2068 from cremno/exc_inspect-convert-mesg-to-string

convert exception message to string
Merge pull request #2069 from cremno/static

various functions should have internal linkage