if non block given

Add Enumerable#minmax_by

Add Enumerable#minmax

Hash#each_{key,value} support return Enumerator if non block given

if non block given

use the same error message

Remove unnecessary mrb_get_args in fiber_switch.

Float#round fix bug for infinity and nan cases

Use bool-macro instead of magic-number

Use FiberError in vm.c.

Implement Fiber#transfer.

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

Add Enumerable#{flat_map, collect_concat}