Delegate irep string free to mrb_gc_free_str
mrb_str_dup use sheard string instead of new string
Merge pull request #2049 from ksss/str-dup

mrb_str_dup return shared string instead of new string
replace memcpy() with a magic number by two assignments as suggested by @cremno; close #2048
Merge branch 'master' of github.com:mruby/mruby