Feature/is equal validator

```
validates :thing, is_equal: { to: 12345 }
validates :thing, is_not_equal: { to: 12345 }
```

is better (faster) than using a Proc with the existing equality validator:

```
validates :thing, equality: { to: Proc.new { 12345 }  }
validates :thing, inequality: { to: Proc.new { 12345 }  }
```
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