exc_inspect: dup file and check mesg only once

reduce RSTRING_PTR usage

mruby-time: two small changes for portability

>If the calendar time cannot be represented, the function returns the
value (time_t)(-1).
http://www.iso-9899.info/n1256.html#7.23.2.3p3

The gem doesn't use the global variables and:
>Local timezone information is used as though localtime() calls tzset().
http://pubs.opengroup.org/onlinepubs/9699919799/functions/localtime.html

Merge branch 'Refactor-numeric.c' of https://github.com/suzukaze/mruby into suzukaze-Refactor-numeric.c

Fix indent in bit_coerce

Refactor fix_rev

Restore arena index since large struct may create many Proc objects; a patch from @take-cheeze; ref #2052

Add Array#uniq! test

Refactor array test to use assert_xxx method

mrb_str_dup return shared string instead of new string