[#857] Clean up SingI1

Description

We don't need multiple SingI1 instances now that QuantifiedConstraints is available, so let's not have them.

Related issue(s)

Resolves #857 (closed) by giving SingI1 a quantified superclass constraint and a single polymorphic instance, and turning its method into a function.

Changes the name of SingI1 to SingIOne to resolve a potential name clash with the similar (but inferior) class in singletons.

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