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()`

The `mrb_env_new()` function is a global function, but it is still treated as an internal function.

Prohibit array changes by "a"/"*" specifier of `mrb_get_args()`

Prohibit array changes by `mrb_get_argv()`

The `mrb_get_argv()` function will now return `const mrb_value *`.
This is because it is difficult for the caller to check if it is a splat argument (array object) and to write-barrier if necessary.

The "a"/"*" specifier of the `mrb_get_args()` function will now return `const mrb_value *`.
This is because it is difficult for the caller to check if it is an array object and write-barrier if necessary.
And it requires calling `mrb_ary_modify()` on the unmodified array object, which is also difficult (this is similar to #5087).

* In explanation of mruby, the expression `build_config.rb` is frequently
  used including official documents, so I think that it will not make sense
  if the file is no longer used.
* The `MRUBY_TARGET` mechanism seems to have little improvement, so I don't
  think it should be changed to avoid unnecessary confusion.
* `MRUBY_TARGET` and `MRuby.targets` represent somewhat different things,
  so using the same term "target" is a bit confusing.

The mechanism that can be written short when using a file under
`build_config` (renamed from `target`) directory remains
(`build_config/${MRUBY_CONFIG}.rb` is used if the path specified
in `MRUBY_CONFIG` doesn't exist).

Ref #5093; close #5085

Fix for #5093

It was once turned off by #4045

Thanks to @YunzheZJU

According to `doc/guides/compile.md`; thanks to @YunzheZJU