Add example for cross-compiling to Nintendo Switch

Add configuration for MinGW cross compilation [ci skip]

along with repeated calls of `strlen()`.

Cross Compiling configuration for the Nintendo Switch, it requires Nintendo SDK

With this cross compiler configuration, all tests for full-core gembox on
Windows on GitHub Actions were successful.

This change also fixes the error caused by `rational.c` that calls
`mrb_complex_eq()`, which had been undefined.

`MRB_INT_MIN` does not have a corresponding positive value.

- define `MRB_TT_COMPLEX`
- change object structure (`struct RComplex`)
- add memory management for `MRB_TT_COMPLEX`
- avoid operator overloading as much as possible
- as a result, performance improved a log
- should work with and without `Rational` defined

- define `MRB_TT_RATIONAL`
- change object structure (`struct RRational`)
- add memory management for `MRB_TT_RATIONAL`
- avoid operator overloading as much as possible
- implement division overloading in C
- as a result, performance improved a lot

Since `mruby` does not have `Bignum`, `Float#divmod` could overflow, so
it will return `Float` values when the divided value does not fit in
`mrb_int`. This behavior will be changed when `Bignum` is introduced to
`mruby` in the future.

`Float::NAN/0` should be `Float::NAN`; ref a0b3378b

#### Before this patch:

```console
$ bin/mruby -e 'p(Float::NAN/0)'
Infinity
```

#### After this patch (same as Ruby):

```console
$ bin/mruby -e 'p(Float::NAN/0)'
NaN
```

If I break out of a block given to `MRuby::Build.new` with `break` or `throw`, I will get a seemingly inexplicable error because the `presym`-related initialization is not done.

```console
% cat build_config1.rb
MRuby::Build.new do
  toolchain
  break
end

% rake CONFIG=build_config1.rb
rake aborted!
external mrbc or mruby-bin-mrbc gem in current('host') or 'host' build is required
/var/tmp/mruby/lib/mruby/build.rb:332:in `mrbcfile'
/var/tmp/mruby/tasks/mrblib.rake:9:in `block in <top (required)>'
/var/tmp/mruby/lib/mruby/build.rb:18:in `instance_eval'
/var/tmp/mruby/lib/mruby/build.rb:18:in `block in each_target'
/var/tmp/mruby/lib/mruby/build.rb:17:in `each'
/var/tmp/mruby/lib/mruby/build.rb:17:in `each_target'
/var/tmp/mruby/tasks/mrblib.rake:1:in `<top (required)>'
/var/tmp/mruby/Rakefile:27:in `load'
/var/tmp/mruby/Rakefile:27:in `<top (required)>'
(See full trace by running task with --trace)
```

If a non-exceptional global jump occurs, it can be initialized by `ensure` to solve this problem.

It used to be return the default value if available, but it should
ignore the default value for behavior consistency. CRuby will adopt
this behavior too in the future. [ruby-bugs:16908]

Fix typo in `AUTHORS` [ci skip]

The issue was reported by @shuujii

Add new pool value type `IREP_TT_BIGINT` and generate integer overflow
error in the VM. In the future, `mruby` will support `Bignum` for
integers bigger than `mrb_int` (probably using `mpz`).