Add helper methods to CustomValue and its implementations

[BoogieMonster1O1]

CustomValue's implementations do not implement equals or hashCode. This causes issues when storing them in lists and maps, where it is impossible to remove CustomValues from a list, or get the value associated with a CustomValue key from a map.
This also affects any DynamicOps relying on CustomValues as FieldDecoders get returned null when reading a CustomValue keyed map.

This makes CustomValue's implementations implement equals, hashCode and toString.
This also adds a stream method to CvObject and CvArray, and a keySet method to CvObject for convenience.

[liach]

The custom value's status as not being value-based should be a bug. @sfPlayer1 What's your view on this whole thing? Will you include the new utilities besides fixing the value-based nature of these classes?

[liach]

@sfPlayer1 Even though the addition of stream methods are actually optional (as they can always stream an iterable), I wonder if we want to treat our custom values as data classes (as opposed to identity objects as they are right now)

[sfPlayer1]

Not sure, it's really just a simple way to expose the data from the mod jsons. Implementing toString has some benefits in the debugger, but uses that depend on equals/hashCode having specific semantics are unclear.