Implement Eq alongside Hash

[benesch]

It turns out implementing Hash alone is not very useful, as
std::collection::HashMap keys are required to implement both Hash and
Eq.

[nickolay]

Would you mind commenting on why Eq makes sense for AST types? Otherwise I’ll get to it later - i’d have to reread the docs before I can say I understand this change.

Not for this PR: have you looked into whether it’s possible to share a common set of #[...] attributes across all of our AST types?

[coveralls]

Pull Request Test Coverage Report for Build 384

• 40 of 40 (100.0%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.239%

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Totals
Change from base Build 383:	0.0%
Covered Lines:	4326
Relevant Lines:	4690

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💛 - Coveralls

[benesch]

Would you mind commenting on why Eq makes sense for AST types? Otherwise I’ll get to it later - i’d have to reread the docs before I can say I understand this change.

You bet! The difference between PartialEq and Eq is that PartialEq does not require reflexivity (a == a for all a), while Eq does. The canonical example is that floats cannot implement Eq because NaN is not equal to itself. (Aside: this isn't an issue for us since we're already using OrderedFloat to provide Hash/Eq/PartialEq/Cmp/PartialCmd on our Value::Float variant.) There's essentially no situation where you don't want Eq if it makes sense, since otherwise you can't use the type as a key in a hash map.

[nickolay]

Ah, right! If only it was called ReflexiveEq, I wouldn’t have so much trouble remembering it. Thanks, this makes perfect sense now.

[benesch]

Awesome, thanks for the review!