Improve `Arbitrary` instance.

[ChickenProp]

Fixes #980. As discussed there, simply changing n <= 0 to n <= 1 gives good generation of strings, bools, numbers and null. Turns out it also forbids [] and {} from being generated, so I added those explicitly to that case.

My understanding is this requires a major version bump, as a change in behavior of an exported function.

[phadej]

Looks good indeed, Yet I'll write some tests to we can actually claim anything about quality of generated values.