test unionized params

[KiaraGrouwstra]

Types aren't values, and keeping types union-safe is a pretty big deal for broader usability. I should actually start testing for that, and ideally documenting it too.

[KiaraGrouwstra]

Known edge-cases to test for:

• union types input: the result of a type function should equal include that of the union of the individual inputs (F<A|B> >= F<A>|F<B>, distributivity)?... though things like Indeterminate and its dependents would definitely not satisfy this, nor e.g. IsUnion... where does this discrepancy stem from? UnionHasKey?
  • which union-proof inputs can take never? seemingly all basic operators minus property access are never-proof. somehow & also failed, thought that's fixed since TS 2.5?
• sub-types: given sub-types of the envisioned input (e.g. 'a' -> 'a' & string), the output should also extend that of the original (for A < B, F<A> < F<B>)
• super-types: given super-types of the envisioned input, the output should also be a super-type of the original (for A > B, F<A> > F<B>)
• prototypes: type functions should be tested for input involving methods on Object.prototype -- that is, for objects try having an explicit toString method, for string input, try toString.