refactor fix_mod

fix unreachable condition
use b

Add functions for safe addition and subtraction

Implement day of week methods.

fix memcpy-param-overlap

Add Hash#keep_if

Refactor read_lineno_record_1() in load.c

Refactor string.c

Fix indents in mrb_str_inspect() of string.c

Use mrb_bool instead of mrb_int  in string.c

Add Hash#invert

Refactoring exception test.

Change to use assert_equal explicitly.

Don't reset ridx,eidx. And fix for #1949

Add document of header searcher of compiler.

Add Hash#flatten

Embedded document

Add Hash comments

Fix typo.

Fix comment of exc_inspect (and adjust spaces)