Implement String#clear
git push origin masterMerge branch 'suzukaze-add-string.clear2'
inline str_discard(); ref #2370
clear shared and nofree flags; ref #2370
move STR_* macros to mruby/string.h with renaming
remove UTF-8 strings from mruby-string-ext test
move String#clear to mruby-string-ext; ref #2370
Add test for end-of-program marker.
Merge pull request #2371 from take-cheeze/end_of_code

Add test for end-of-program marker.
revert #2368 due to #2369
do not ignore negative characters (e.g. EOF and partial EOF); fix #2361 fix #2369
Add test for end-of-program marker.
Merge branch 'master' of github.com:mruby/mruby
Merge branch 'master' into iij