Avoid comparing pointers and integers

Makes `mrb_any_to_s()` accept an object whose class is `NULL`

When using `mrb_any_to_s()` for debugging purposes, giving an object
whose class is `NULL` no longer causes a SIGSEGV and no crash.
This is achieved by making `mrb_class_name()` and `mrb_str_cat_cstr()`
null safe.

`assert()` can be completely removed when `NDEBUG` is set.

Ref #4982 #4983

The behavior is different from CRuby, but we believe this is a right
behavior for mruby, which only supports either ASCII or UTF-8
exclusively; fix #4983, ref #4982

```
$ printf '\xe3\x81' | ruby -e 'p STDIN.readchar'
"\xE3\x81"
```

```
$ printf '\xe3\x81' | mruby -e 'p STDIN.readchar'
"\xE3"
```

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.