Implement Struct#to_a and Struct#values .
Add Array#select_bang
Fix MSVC warnings for numeric.c
rename `mrb_str_buf_append` to `mrb_str_cat_str`

The new name is better and less confusing, because:
  - `mrb_str_append` calls `mrb_str_to_str` and this function doesn't
  - `mrb_str_buf_append` _is_ `mrb_str_cat` for `mrb_value` strings
Merge pull request #2138 from take-cheeze/struct_to_a

Implement Struct#to_a and Struct#values .
Merge pull request #2139 from suzukaze/add-array.select_bang

Add Array#select_bang
Merge pull request #2141 from kyab/fix_msvc_warning_numeric.c

Fix MSVC warnings for numeric.c
Merge pull request #2142 from cremno/rename-mrb_str_buf_append

rename `mrb_str_buf_append` to `mrb_str_cat_str`