Rename "fuzz" and "fuzzer" to something else

[Janiczek]

Fuzzing is related to property-based testing but is its own separate thing.

We should not confuse people with weird naming (compared to the rest of the world).

Our "fuzz tests" are really property-based tests
Our "fuzzers" could be(?) generators, arbitraries, parsers. Maybe some other community found a better name. (Hypothesis has Strategies but the author hates that name in retrospect.)

This would need a major version bump, as well as changes to the runners.