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`