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}

Fix behavior of Comparable methods when <=> return nil

when <=> return nil

Reword mention of mrbtest in README

add tests for TrueClass

Add Enumerable#count comment

Fix typos in README.md and doc/compile/README.md

reorder and add tests for 15.2.5.1