* Reject exceptional input to `isConstructor`
Prior to this commit, the `isConstructor` harness function would return
`false` when invoked with a value that lacked a [[Call]] internal
method. While it's true that such values are not constructors, there are
no tests which benefit from using `isConstructor` to make such an
assertion.
Extend `isConstructor` to throw an error when invoked with a
non-function object. Update a test which was misleadingly invoking the
function with the value `undefined`.
* fixup! Reject exceptional input to `isConstructor`
