Have a mode that crashes on all panics

[jakubadamw]

So with #6 and #7 merged in rutenspitz no longer crashes in a fuzz cycle on a panic when both the implementation and the model panic in accordance. This change in behaviour is fine, since the main purpose of this crate is to observe the equivalence of two models. But I think it may be useful to use it also to find panics on malformed input (well, a series of operations to a model). This is exactly what the url::Url fuzzer did, although it did in a rather convoluted way by using url::Url as the supposed doppelganger of url::Url itself.

Rather than doing that we should have a mode which turns off all comparisons, essentially always using https://github.com/jakubadamw/rutenspitz/blob/ff2c1b27cf2adba4d354dcf18e2877a9fc9c4ee5/lib/src/lib.rs#L482-L486 for individual operation tests, without using (and requiring) a reference model at all.

This is perhaps somewhat out of scope given the name of this crate but nonetheless seems like a useful extension.