Fuzzers should exhaustively check all values of a type if possible/reasonable

We've been talking with @gampleman about this a few times, and I believe I have a rough plan of attack.

Expand the Fuzzer definition, add an `exhaustive : Maybe (() -> List a)` field inside or make a sum type:
```elm
type Fuzzer
= Random (PRNG -> GenResult a)
| Exhaustive (() -> List a)
```

For certain fuzzers (`bool`, `unit`, `intRange` in case the range is below some threshold, etc.) this would be populated with `\() -> [False, True]` etc. and would be used _instead of_ randomly generating values.

For other fuzzers it would be disabled (`float`, `int`, `string`, `list` etc.).

Then there's a class of fuzzers which preserve the exhaustive mode if all their children are exhaustive: `map`, `lazy`, `tuple`, `triple`, `oneOf`, etc.

----

We would still honor the `runs : Int` config, but we'd warn if exhaustiveness was possible but above the threshold: something like

> NOTE: Fuzzer for this test can be checked exhaustively (324 cases). To enable the exhaustive check increase your `--runs` configuration value.