Add API `mrb_atexit()`.
`mrb_final_mrbgems` will be called as mrb_state atexit function.
Maybe useful in #1844.
update Kernel#raise/fail argument description
add nil#to_h; ref #2348
Merge branch 'atexit' of https://github.com/take-cheeze/mruby into take-cheeze-atexit
git push origin masterMerge branch 'take-cheeze-atexit'
remove eval (that only raises NotImplementedError) from the core; ref #2354
remove mrblib/print.rb that only raise NotImplementedError; ref #2354
move IS_EVSTR macro that should be private to src/string.c