Equality implementation

[amis92]

I thought we agreed to use direct operator equality/valuetuple equality. That might come later.

Originally posted by @amis92 in https://github.com/amis92/RecordGenerator/pull/56

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There are a couple of areas where we can certainly improve. Let's list them and discuss them.

[amis92]

Found a problem as well, GeneratedCodeAttribute isn't applied on generated operator overloads (==/!=)

edit: moved to #77

[amis92]

operator == should defer to direct left.Equals(right) call instead of using EqualityComparer.

This could be an optimization when we know we implement ObjectEquals and/or EquatableEquals. If not, we could probably check if the input class already implements either and if so, use that as well.

Prioritization of calls should be:

1. IEquatable<T>.Equals(T) if interface is implemented (by us or not)
2. object.Equals(object) if overridden
3. EqualityComparer<T>.Equals(T, T) as a fallback