Heterogeneous collection types implemented with row polymorphism.

This library provides additional data types that are parameterized by polymorphic rows. That is to say, they are type constructors of kind Row Type -> Type. Some examples of existing data structures in the purescript ecosystem that fit this pattern include:

• Record Extensible polymorphic product types. They have size N for an N-row parameter.
• Variant Extensible polymorphic sum types. They have size 1 for an N-row parameter.

This library adds the following additional types:

• HMap Extensible polymorphic products with optional membership. They have size [0, N] for an N-row parameter. These can be thought of as maps with finite keys and value types that are determined by the keys. Another way to think of them is as records in which every field is optional.
• NonEmptyHMap A non-empty version of an HMap. They have size [1, N] for an N-row parameter. An interesting way to think about these is as a composition of Variants and Records, where each key in the Variant is associated with a Tuple that includes a value of the type for the Variant key, and an HMap with the Variant key removed from the row.
• Null An empty collection. They have size 0 for an N-row parameter. Mostly included for completeness.

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