Remove redundant NULL checks.

mrb_malloc causes an exception when memory was empty.

add new function mrb_toplevel_run to prevent running through C function boudaries on exceptions; close #1942

Fix unnecessary string macro

Use mrb_int instead of int

Enumerable#find_index

Use mrb_int instead of int

Use mrb_int instead of int

mrb_str_pool add string capa

src/string.c RESIZE_CAPA use safe name variables

* add multi value support
* `each` method may not rewind the sequence so that `cycle` should save elements

Add Enumerable#cycle

This has a possibility of naming conflicts

Add Enumerable#reverse_each

Implement \u notation for strings and regexes.