fix memcpy-param-overlap
Add Hash#invert
Refactor mrb_str_capitalize_bang()
Refactor mrb_str_downcase_bang()
Refactor mrb_str_split_m()
Refactor mrb_str_upcase_bang
Refacotr mrb_str_substr()
Refactor mrb_str_hash()
Fix indents in mrb_str_inspect() of string.c
Refactor read_lineno_record_1() in load.c
Merge pull request #2192 from suzukaze/add-hash.invert

Add Hash#invert
Merge pull request #2193 from suzukaze/refactor-string.c

Use mrb_bool instead of mrb_int  in string.c
Merge pull request #2195 from suzukaze/fix-indent-string.c

Fix indents in mrb_str_inspect() of string.c
Add Hash#keep_if
Merge pull request #2194 from suzukaze/refactor-string.c2

Refactor string.c
Merge pull request #2196 from suzukaze/refactor-load.c

Refactor read_lineno_record_1() in load.c
Merge pull request #2198 from suzukaze/add-hash.keep_if

Add Hash#keep_if
temporary macros for checked arithmetic functions
add function for checked mrb_int addition
add function for checked mrb_int subtraction