Add variation_count on classes

Count how many "variations" each class creates. A "variation" is a a
unique ordering of instance variables on a particular class. This can
also be thought of as a branch in the shape tree.

For example, the following Foo class will have 2 variations:

```ruby
class Foo ; end

Foo.new.instance_variable_set(:@a, 1) # case 1: creates one variation
Foo.new.instance_variable_set(:@b, 1) # case 2: creates another variation

foo = Foo.new
foo.instance_variable_set(:@a, 1) # does not create a new variation
foo.instance_variable_set(:@b, 1) # does not create a new variation (a continuation of the variation in case 1)
```

We will use this number to limit the amount of shapes that a class can
create and fallback to using a hash iv lookup.

Co-Authored-By: Aaron Patterson <tenderlove@ruby-lang.org>

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