This fix only effective when `MRB_UTF8_STRING` is set.

- mrb_utf8len() - returns the size of a UTF-8 char (in bytes)
- mrb_utf8_strlen() - returns the length of a UTF-8 string (in char)

The bug was introduced by #4712. The `getc' problem resurrected.
It should be addressed soon.

Test mruby-io in tmpdir when AF_UNIX cannot be created on cwd

This reverts commit 682a31f9.
Unfortunately, I couldn't run newer `bison` on TravisCI. Maybe next
time.

If it doesn't work, I will revert the last update to `parse.y`.

Recent `bison` warns for `%pure-parser`. We kept it since MacOS only
provide ancient `bison`, but the warning is noisy and there's no hope
that Apple will upgrade `bison`. MacOS users must install the newer
version of `bison`, by typing `brew install bison` for example.

Note that `brew` does not overwrite the `bison` execution path
automatically, so you need to update your `.bash_profile` as instructed
by `brew`.

Integrate class `EnsureYieldBreak` under test

Reduce `struct mrb_context`

Remove unused `mruby-error` in `mruby-bin-mruby`

Support `undef` for `mrb_ary_splice()` instead of `[]`

Removed comments mentioning `MRB_WITHOUT_FLOAT`; ref 2e661e82

The comment is for messages when `MRB_INT16` and `MRB_WITHOUT_FLOAT` are
specified at the same time.
The comment itself is no longer needed now that `MRB_INT16` is gone.

When removing elements from an array, it is possible to avoid creating
an empty array.

Before this patch:

```c
mrb_ary_splice(mrb, ary, head, len, mrb_ary_new(mrb));
```

After this patch:

```c
mrb_ary_splice(mrb, ary, head, len, mrb_undef_value());
```

Thank you @shuujii to additional report on #4973

The difference between `mrb_singleton_class` and `mrb_singleton_class_ptr`:

- `mrb_singleton_class_ptr` returns `struct RClass*`.
- `mrb_singleton_class_ptr` returns `NULL` on immediate values where
  `mrb_singleton_class` raises exceptions.

The tranpoline code in 6a0b68f8 was wrong; reverted.

In ISO, those methods should raise `TypeError`, but the spec has been
changed.  The change was discussed in [Feature#12979].

Check the file descriptor with `IO#initialize`; resolve #4966

Fix `MRB_TT_CPTR` object with `MRB_NAN_BOXING`

Support `MRB_DISABLE_STDIO` for mruby-pack; ref #4954

 Support `MRB_DISABLE_STDIO` for mruby-sprintf; ref #4954

Fixed a build error in mruby-config

Previously, if `MRB_NAN_BOXING` is defined, for example,
`mrb_cptr_value()` could not keep an odd address.

If it is `MRB_32BIT`, it can be embedded in `NaN` as it is.

If it is `MRB_64BIT`, some operations are shared with `MRB_WORD_BOXING`.
In this case, the MRB_API function `mrb_nan_boxing_cptr_value()` is
defined.