The ISO standard does not include implicit type conversion using
`to_int`. This implicit conversion often causes vulnerability.
There will be no more attacks like #4120.

In addition, we have added internal convenience method `__to_int` which
does type check and conversion (from floats).

The issue #3920 was fixed but the fundamental flaw of lack of stack
depth check along with fibers still remains, even though it's not
easy to cause the issue.  Use `MRB_GC_FIXED_ARENA` to avoid the issue
for workaround.

After this patch, `obj.to_enum` raises `ArgumentError` if the object
does not respond to the enumerating method. This is incompatible to
CRuby but I think this behavior is better and CRuby should be updated
to behave like this.

The GC may occur between `sg_shift` and `mrb_assoc_new`, in which
case `key` and `value` could be freed even tough they are still alive.

The issue is found and fixed by https://hackerone.com/hexodus

The saving `pc` position should be beginning of the instruction.
But after `mruby 2.0` byte code modification, the `pc` variable
points the beginning of the next instruction. We save the previous
position in a local variable `pc0`.

Line number information in a compiled file was wrong.

This patch slightly reduce memory consumption (2% for my test).

The issue is reported by `https://hackerone.com/dgaletic`.

Wrong pool data length for negative floating value in a mrb file.

It should be done by planned embedded symbols.

Reduce instruction size

Since our new VM instruction is byte based, there's no need to endian
conversion of instruction sequences.

By dffa203d that reclaim `env` objects from heaps, there's more chance
for `env` objects referenced from fibers may be freed from heap pages.

Fixed a bug in INIT_DISPATCH for non direct threading; fix #4153

Add debug flag to `MRuby::Build`