This issue was first reported by https://hackerone.com/geeknik
The fix was proposed by @titanous

need to find out real memory bug that appears in full-debug/mrbtest

GitHub: fix #2902, #2917

The current implementation traverses stack to retrieve backtrace. But
stack will be changed when some operations are occurred. It means that
backtrace may be broken after some operations.

This change (1) saves the minimum information to retrieve backtrace when
exception is raised and (2) restores backtrace from the minimum
information when backtrace is needed. It reduces overhead for creating
backtrace Ruby objects.

The space for the minimum information is reused by multiple
exceptions. So memory allocation isn't occurred for each exception.

Sometimes it is very useful just return from mrb_close if NULL is
passed as mrb.  This is the same spirit of free(3), which just does
nothing if NULL is passed.

When DISABLE_GEMS is not defined and
a return value of mrb_open_core() is NULL,
mrb_open_allocf() may cause SEGV.

Since some use it as `mrb_state` associated user data.

Adds following macros:
* MRB_FIXED_STATE_ATEXIT_STACK (not defined by default)
  * When defined enables fixed state atexit stack.
* MRB_FIXED_STATE_ATEXIT_STACK_SIZE (default value: 5)
  * This macro will be ignored when `MRB_FIXED_STATE_ATEXIT_STACK` isn't defined.
  * When `mrb_state_atexit` is called more than this value it will raise runtime error.

rename mrb_atexit to mrb_state_atexit to clarify a func will be called at the end of state, not process; ref #2211

`mrb_final_mrbgems` will be called as mrb_state atexit function.
Maybe useful in #1844.

mrb_malloc causes an exception when memory was empty.

use flags 4 for *this object is embed*
use flags 8~64 for *embed string length*

- reduce compile time by a little bit (full-core: ~0.7s for me)
- thanks to 'include-what-you-use' for some help
- include Standard C header files before any other (coding style)

You will not see "arena overflow" error anymore, but I encourage gem authors
to check your gems with MRB_GC_FIXED_ARENA to avoid memory broat.