- `MRB_64BIT`: the size of a pointer is 64 bits
- `MRB_INT64`: the size of `mrb_int` is 64 bits

Type `mrbc -S -B<init> -o<outfile> <rbfiles...>` to generate the C
source code that holds compiled `mrb_irep`.

Appending the following code to the bottom of the generated code,
`mruby` executes the compiled code:

```C
int
main()
{
  mrb_state *mrb = mrb_open();
  struct RProc *p = mrb_proc_new(mrb, &init_irep);
  mrb_vm_run(mrb, p, mrb_top_self(mrb), 0);
  mrb_close(mrb);
  return 0;
}
```

Eventually static compile should use this representation, instead
of `uint8_t` array that holds `mrb` data, so that we can skip
interpreting `mrb` data.

We no longer need 4 bytes alignment after we moved to the byte oriented
instructions.

Changes:
- `pool format is completely replaced
  - supported types: `STR`, `INT32`, `INT64`, `FLOAT`
  - `FLOAT` may be replaced by binary representation in the future
- insert `NUL` after string literals in `mrb` files
- `irep->pool` no longer store values in `mrb_value`
- instead it stores in `mrb_pool_value`
- less allocation
- `mrb_irep` can be stored in ROM

That stands for "local variable information".

But we need more work:
- recursive `irep` dump (`irep->reps`)
- pool values dump (`irep->pool`)

Since it's not supported on VC without `/std:c++latest`. That means it
doesn't work for `cxx_api` build on Windows VC.

The option is not available in the old version of Ruby.

- `pool`
- `syms`
- `reps`

You have to specify `TARGET` to specify a configuration, e.g.

```
rake TARGET=host-debug all test
```

When you port `mruby` to a new configuration:
1. copy an existing configuration under `target` directory
2. modify the new configuration file
3. build using the new configuration
4. send PR if you please

* Disable automatic update and clean up on `brew install` (install time
  160 sec -> 5 sec).
* Avoid using deprecated keys.