New instructions:
  * OP_LOADL16
  * OP_LOADSYM16
  * OP_STRING16

Size of pools, symbols are `int16_t` but offset representation in the
bytecode was 8 bits. Size of child `irep` array is `int16_t`, too but
this change does not address it.

`minirake` was still used in many occasions.
This reverts commit 3847c7eb.

Since `R-assignment` in CRuby is abandoned.  Single-line pattern matching
in `mruby` only matches single local variable at the moment. Currently
it works as a right assignment to a local variable. It will be enhanced
in the future.

Fix parallel build

When `rake -m` and so on are used to build in parallel, building may be
started before presym files are generated. Then, for example, the following
error occurs and this issue is fixed.

```console
In file included from /Users/shuujii/mruby/mruby/include/mruby.h:92:
/mruby/mruby/include/mruby/presym.h:16:10: fatal error: '../build/presym.inc' file not found
#include <../build/presym.inc>
         ^~~~~~~~~~~~~~~~~~~~~
```

ossfuzz: Fix mruby include paths

Add `preview` to version.

If you confirm the gem is cdump safe, add `spec.enable_cdump` in
`mrbgem.rake` file. Some external gems e.g. `mruby-mgem-template` do
not work with cdump.

Always use `mrblib` and 'src` for directory names.

Fix typo in `Range#last`

Fix typo in `doc/guides/compile.md` [ci skip]

I changed to use `full-core` gembox for debugging. It should have been
reverted before `mruby3` branch was merged.

It was relative from the directory of the build configuration file, but
the document says it's relative from `MRUBY_ROOT`. When the default
configuration file was `MRUBY_ROOT/build_config.rb`, it was OK for most
of the cases.

In addition, update the documents referring `build_config.rb` which is
no longer used. The new `build_config.rb` describes the new configuration
structure in the comment.

This reduce unnecessary calls of `mrb_funcall()`.

Revert `MRUBY_TARGET` mechanism; ref #5096

Fixed build with `conf.enable_cxx_exception`

Fix `mrb_obj_id` to `Float`

The problem was manifested by commit 5069fb15.

Reorganize `env_new()` as `mrb_env_new()`