Ivars will longer be the only thing stored inline
via shapes, so keeping the `iv_index` and `ivptr` names
would be confusing.

Instance variables won't be the only thing stored inline
via shapes, so keeping the `ivptr` name would be confusing.

`field` encompass anything that can be stored in a VALUE array.

Similarly, `gen_ivtbl` becomes `gen_fields_tbl`.

This commit moves the classpath (and tmp_classpath) from instance
variables to the rb_classext_t. This improves performance as we no
longer need to set an instance variable when assigning a classpath to
a class.

I benchmarked with the following script:

```ruby
name = :MyClass

puts(Benchmark.measure do
  10_000_000.times do |i|
    Object.const_set(name, Class.new)
    Object.send(:remove_const, name)
  end
end)
```

Before this patch:

```
  5.440119   0.025264   5.465383 (  5.467105)
```

After this patch:

```
  4.889646   0.028325   4.917971 (  4.942678)
```