With the following code:

```ruby
object = []
object.singleton_class
object.freeze
object.instance_variable_set(:@a, 42)
```

The previous error message was:

```
can't modify frozen #<Class:#<Array:0x00000631d1308f78>>: []
```

With this change, the error message is:

```
can't modify frozen Array: []
```

Since we show the inspect value of the affected object, I think
including the singleton class instead of the actual class if it
exists makes the error message harder to understand.

Visual C++ 2005 (8.0):
- _MSC_VER: 1400
- MSVCRT_VERSION: 80

Inspired by 42ba82424d908c290a4a34ced8853f0a403b734b, I looked for other
occurrences of "the the".

ec is unavailable on timer thread, for instance.

The `p->field = rb_gc_location(p->field)` isn't ideal because it means all
references are rewritten on compaction, regardless of whether the referenced
object has moved. This isn't good for caches nor for Copy-on-Write.

`rb_gc_mark_and_move` avoid needless writes, and most of the time allow to
have a single function for both marking and updating references.