Prevent accepting String value for mrb_get_args("i")

Accepting String value for mrb_get_args("i")
causes unintentional expression.

  * `"str" * "3"` => `"strstrstr"`

This change prevents String->Integer conversion for mrb_get_args("i")
and fixes above problem.

See also:
https://github.com/mruby/mruby/issues/1903
reduce object allocation in __update
implement Fiber#transfer
make Fiber#transfer compatible with CRuby
Merge pull request #1904 from ktaobo/get-arg-int

Prevent accepting String value for mrb_get_args("i")
Merge branch 'master' of github.com:mruby/mruby
Merge pull request #1900 from take-cheeze/fiber_transfer

Implement Fiber#transfer.
Merge branch 'master' of github.com:mruby/mruby
add const qualifier to pacify warning; ref #1900
should test Enumerable#first; close #1905
use mrb_get_args("i")