BigArray/ByteArray improvements

[hhoffstaette]

We could use some extensions to BigArray/ByteArray:

• copy() - similar to System.arrayCopy()
• equals()/hashCode()

The latter is mostly driven by PagedBytesReference which currently materializes the pages into one big byte[], which is unnecessary.

@jpountz Suggestions where and how to add equals/hashCode?

[jpountz]

I think it would make sense to have these methods implemented as helper methods in BigArrays (quite similarly to java.util.Arrays for simple byte[] arrays)?

[s1monw]

+1 to add helper methods like BytesRef.deepcopy() do make sure it's an actual copy and shallowCopy etc

[hhoffstaette]

As mentioned in the PR #5435 we reduced the scope of this issue to equals/hashCode for now.