Pull EnumValue into Enum class

Basically removed every instance of the word "value" except in the Numeric
extension and the deprecated instance method.

Also fixed an issue where `Enum.fetch` would return the enum you passed in
if it was any defined Enum, regardless of if it was defined under that type:

```ruby
class Foo < Protobuf::Enum
  define :ONE, 1
end

class Bar < Protobuf::Enum
  define :TWO, 2
end

Foo.fetch(Bar::TWO) == Bar::TWO # => this is a bug, fixed now
```