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.

Because some changes have been overridden.

I sometimes see Bison related problems in setting up build environments.
Therefore to remove Bison from build time dependencies, add `y.tab.c`
generated by Bison to the repository.

The reduction of dependency at build time also reduces the labor and time
for setup and installation in CI.

In addition, a path in `#line` directive is converted to a relative path so
that its path is constant regardless of development environments.

`Windows-MinGW` and `Windows-VC` also requires updates.

- `mrb_convert_type`
- `mrb_check_convert_type`

Those function no longer take `tname` string representation of desired
type, and take method symbols instead of `const char*` names. This is
incompatible change. I hope no third-party gems use those functions.

- mrb_define_class_id
- mrb_define_module_id
- mrb_define_method_id
- mrb_define_singleton_method_id
- mrb_define_module_function_id
- mrb_define_const_id
- mrb_undef_method_id
- mrb_undef_class_method_id
- mrb_class_defined_id
- mrb_class_get_id
- mrb_class_defined_under_id
- mrb_class_get_under_id
- mrb_module_get_id
- mrb_module_get_under_id
- mrb_define_class_under_id
- mrb_define_module_under_id
- mrb_exc_get_id

- :

Except for support files e.g. `mruby-test/driver.c`, which are not
target of symbol collection via `rake gensym`.

`mrb_funcall_id()` takes `mrb_sym` instead of `char*` for a method name.
You can use `MRB_SYM()`/`MRB_QSYM()` to specify the method to call.