- `OP_EXCEPT` checks if `mrb->exc` is `NULL`, `MRB_TT_EXCEPTION` or
  `MRB_TT_BREAK`.
  If `mrb->exc` is `NULL`, it will be replaced with `nil`.

- If `OP_RAISE` is `nil`, it does nothing and the immediately
  following instruction is executed (like `OP_NOP`).
  Also, in case of `RBreak` object, it moves to the processing for
  `break`.
  With this change, the instruction name is changed from
  `OP_RAISE` to `OP_RAISEIF`.

The catch handler table is combined with iseq block.
This is to prevent the structure from growing by adding a field for the
catch handler table to the `mrb_irep` structure.

"iseq block" and "catch handler table":
  [number of catch handler table (2 bytes)]
  [number of byte code (4 bytes)]
  [iseq (any bytes)]
  [catch handlers (multiple of 7 bytes)]

catch handler:
  [catch type (1 byte)]
  [begin offset (2 bytes)]
  [end offset (2 bytes)]
  [target offset (2 bytes)]

catch type: enum mrb_catch_type (0 = rescue, 1 = ensure)
begin offset: Includes the specified instruction address
end offset: Does not include the specified instruction address
target offset: replaces pc with the specified instruction address

This table is not expanded by `read_irep_record_1()`.
The necessary elements are expanded one by one when used.

Probably I misunderstand strict aliasing rule of C++. The fix in
4c001673 was other way around.

Since some compiler complains when we `typedef` `mrb_value`.

Clearing all method cache using `memset` is faster than conditional
clear by method id.

You don't need pointer tweaking on 32bit architecture, where pointers
fit in 32bit (lower half of mrb_value).

Tested only on Linux. You need to install 32bit relate libraries,
e.g. libc6-dev-i386.

On 64bit platforms: `MRB_NAN_BOXING`
On 32bit platforms: `MRB_WORD_BOXING`
On debugging: `MRB_NO_BOXING`

Since it's an internal static function.

When the array is very big, the simpler `mrb_write_barrier` causes
calling `gc_mark_children` for big arrays repeatedly. That would hinder
performance very badly.

Which is caused by `MRB_NAN_BOXING`.

Introduced `MRB_NO_METHOD_CACHE` which is inverse of `MRB_METHOD_CACHE`
that should be enabled intestinally. In addition, the default cache is
made bigger (128 -> 256).

The minor versions should be upper compatible. So mere opcode, section
addition can be done without breaking compiled binary.

- no OP_EXT_ anymore
- OP_LOADI16 in right position

[ruby-bugs:15921]

They are included from `mruby.h` anyway, and including it ahead can
cause some errors regarding `INT32_MAX` etc. with C++ compiler.

This combination is too hard to support. Difference between C and C++ is
too big with VisualC++. GCC and clang are OK.