Add test for backquote.

* Implement code generation of NODE_DXSTR.
* Fix NOVAL NODE_XSTR.

Implement Kernel#__method__

Implement Object#tap

add String#oct

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