Enable MRB_METHOD_T_STRUCT by default on 32bit GUN/Linux

Because we can't use the highest 2 bits of function pointers.

Avoid changing directory in `mruby-io` test

Refine `.travis.yml`

* Disable automatic update and clean up on `brew install` (install time
  160 sec -> 5 sec).
* Avoid using deprecated keys.

Sorting the list of gems at build time by name

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.

Printing them in sorted order makes it easier to find the desired gem.
But it has come to completely ignore the dependency.

`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`