Implement String#hex

Use size_t instead of int

Use CRuby compatible key in ObjectSpace.count_objects .

Adapt TypeError Message for String to (Integer|Float) Conversion

MRI:
"str" * "3"
=> TypeError: no implicit conversion of String into Integer

mruby:
"str" * "3"
=> TypeError: String can't be coerced into int

Catch up to latest Arduino IDE

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