[Bug #21718]

Otherwise objects that don't define it, but define a fairly liberal
`method_missing` method will run into errors that are hard to understand:

```ruby
class Foo
  def method_missing(name, ...)
    name
  end
end

p Foo.new.inspect
```

```
'Kernel#inspect': wrong argument type Symbol (expected Array) (TypeError)
	from ../test.rb:7:in '<main>'
```

It is already declared as:
> This behavior is slated to be removed in Ruby 4.0

This changeset suppresses backtrace locations like
`<internal:array>:211` as much as possible.

Before the patch:
```
$ ruby -e '[1].fetch_values(42)'
<internal:array>:211:in 'Array#fetch': index 42 outside of array bounds: -1...1 (IndexError)
        from <internal:array>:211:in 'block in Array#fetch_values'
        from <internal:array>:211:in 'Array#map!'
        from <internal:array>:211:in 'Array#fetch_values'
        from -e:1:in '<main>'
```

After the patch:
```
$ ./miniruby -e '[1].fetch_values(42)'
-e:1:in 'Array#fetch_values': index 42 outside of array bounds: -1...1 (IndexError)
        from -e:1:in '<main>'
```

Specifically:

* The special backtrace handling of BUILTIN_ATTR_C_TRACE is now always
  applied to frames with `<internal:...>`.
* When multiple consecutive internal frames appear, all but the bottom
  (caller-side) frame are removed.

[Misc #20968]

Make Kernel#inspect ask which instance variables should be dumped by
the result of `#instance_variables_to_inspect`.

Co-Authored-By: Jean Boussier <byroot@ruby-lang.org>