Implement runtime support for non-exhaustive enums in Java

[Mingun]

Runtime changes for kaitai-io/kaitai_struct_compiler#288

[generalmimon]

I don't think it's beneficial to provide this method in the interface at all if it only makes sense for the Known case. It just seems to encourage null pointer exceptions in user applications.

If the user truly needs a known enum value name, they should first make sure that they really have a known enum value using the instanceof operator. After doing that, they can safely use a type cast to get access to all methods that Java enums have, including name(). I see nothing wrong with this approach.

If they just want to convert the enum value to human readable representation, the Unknown class should actually override Object.toString() so that it shows the integer value it represents (e.g. "42") instead of something like "Unknown@...". Then toString() could be used as a better replacement for name() if the user doesn't want to care if they have a known or unknown enum value.

Speaking of overriding Object methods, Object.toString() is not the only method that should be overridden, as @dgelessus mentioned in kaitai-io/kaitai_struct#778 (comment):

public class UnknownCompressionAlgorithm implements ICompressionAlgorithm {
    ...
    // Insert equals, hashCode, toString, etc. boilerplate here,
    // which need to be implemented manually, unlike in enums.
}

equals and hashCode certainly makes sense - we don't want to use reference equality for Unknown objects. If two different Unknown objects represent the same integer value, then they are obviously equal.

[Mingun]

toString is implemented by generator as <kaitai-java-enum-name>(<enum-long-value>).
equals now also implemented by generator (because we do not want that different classes be equal).

hashCode now implemented here as Long.hashCode(this.id).

[generalmimon]

I don't think it makes sense to include snippets of generated code in runtime library docstrings. I would just remove this.

[Mingun]

That code should help coders to understand what structure expect even when they do not see any generated file, but ok. Removed.

[generalmimon]

Suggested change

	* Interface, implemented by every enum class generated by Kaitai Struct Compiler,
	* which provides assess to the underlying integer value.
	* Interface implemented by every enum class generated by Kaitai Struct compiler,
	* which provides access to the underlying integer value.

I don't really see the point of the "which provides access to the underlying integer value" part, though. It doesn't seem to be a defining characteristic of enums, more like a (obvious) sub-feature. It's probably better to delete it and just leave it at "Interface implemented by every enum class generated by Kaitai Struct compiler."

[Mingun]

I want to have some words to describe not only what it that, but also why it is used. I removed that sentence, but would be glad to hear what can replace it.

[Mingun]

I implemented hashCode(), removed name() and fix documentation. Corresponding equals is generated in each enum by generator.