add const qualifier to pacify warning; ref #1900
should test Enumerable#first; close #1905
use mrb_get_args("i")
use FiberError in vm.c
Use bool-macro instead of magic-number
Add Enumerable#max_by
Support mulitple values in Enumerable#max_by
Merge pull request #1910 from take-cheeze/fiber_error

Use FiberError in vm.c.
Merge pull request #1911 from cubicdaiya/issues/use_bool_macro

Use bool-macro instead of magic-number
fix multiple value support of max_by; ref #1912
wrap expr with paren
add Enumerable#min_by as well; ref #1912
Merge branch 'master' of github.com:mruby/mruby
Merge pull request #1907 from ksss/float-round

Float#round fix bug for infinity and nan cases
remove unnecessary mrb_get_args in fiber_switch
use the same error message
Merge pull request #1913 from take-cheeze/fiber_unnecessary_get_args

Remove unnecessary mrb_get_args in fiber_switch.
Merge pull request #1914 from stomar/struct-emsg

use the same error message
Add Enumerable#minmax
Support multiple values in Enumerable#minmax