Add missing hash, eql?, == to various node classes

[MaxLap]

Some of the nodes classes are missing either one or many of the common comparison methods #hash, #eql? and #==.

This makes comparision and working with the ast sometimes painful, as equality or operations likes array differences (which uses a hash behind the scene) produces unexpected results.

A test has been added that ensures that every descendants of Node:

• have all 3 methods
• that all 3 methods were defined from the same class
• that the class defining all 3 is also a descendant of Node, to avoid the default ones that rely on identity only

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @sgrif (or someone else) soon.

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[sgrif]

These objects aren't generally meant to be used as hash keys. Can you give an example of where this is actually causing an issue in Rails or another consumer of Arel?

[MaxLap]

Well, most of the nodes already define those methods, so i'm assuming there is already some kind of use case.

If you have an array of nodes, and you want to use array methods like #delete, you need equality to work correctly. If you want to use #- or #uniq, then you need to have the hash and eql? since that's what ruby uses behind the scene. So this isn't just about being used as hash key directly.

An explicit use case would be to be able to remove duplicate nodes from an array of nodes.

[sgrif]

I suppose there's enough places already defining these to justify it.