Negative integer `>-65535` had wrong value, e,g, `p(-40550)` printed
`4294926746` since Nov. 2020, sigh.

Also avoid using `uint64_t`.

Fix regression for squiggly heredocs

Refine "wrong number of arguments" message in `mrb_get_args`

#### Before this patch:

```ruby
__send__         #=> wrong number of arguments
{}.default(1,2)  #=> wrong number of arguments
```

#### After this patch:

```ruby
__send__         #=> wrong number of arguments (given 0, expected 1+)
{}.default(1,2)  #=> wrong number of arguments (given 2, expected 0..1)
```

Update copyright year for 2021

Removed unusable `GC.test`

The substance of the method was removed in commit 15ceb35e.

Refine error message from `mrb_get_arg1`

#### Before this patch:

```console
$ bin/mruby -e '{}.key?'
trace (most recent call last):
-e:1: wrong number of arguments (ArgumentError)
```

#### After this patch:

```console
$ bin/mruby -e '{}.key?'
trace (most recent call last):
-e:1: wrong number of arguments (given 0, expected 1) (ArgumentError)
```

Fix mixed indentation and escaped tabs in squiggly heredocs

Fix for empty lines in squiggly heredocs

Add `Hash#except` [Ruby 3.0]

Add support for squiggly heredocs

Improves out of memory messages for `eval`

The comment says that if `mrb_parse_nstring()` returns `NULL`, it is only out of memory.
I'm worried about compatibility if I set the exception class to `NoMemoryError`, so it's still `RuntimeError`.

Create CODEOWNERS file

Add example for cross-compiling to Nintendo GameBoyAdvance

- You can use a CODEOWNERS file to define individuals or teams that are responsible for code in a repository.
- Code owners are automatically requested for review when someone opens a pull request that modifies code that they own. Code owners are not automatically requested to review draft pull requests.
- https://docs.github.com/en/free-pro-team@latest/github/creating-cloning-and-archiving-repositories/about-code-owners
- This creates a bit more automation so that after you merge this PR, next time someone submits a PR @matz will be automatically assigned as the PR reviewer as CODEOWNER
- CODEOWNERS can even be assigned directories or file types

refactor: fix Rake link and fix missing HTTPS on links

Fix typo in `.github/workflows/build.yml` [skip travis][skip appveyor]

Use `git ls-files` instead of `find` in `spell-checker.yml` [ci skip]