MSRV Policy/Disable arbitrary via feature

[TheBlueMatt]

It seems the update to arbitrary 1.0 requires const generics, which were stabilized only incredibly recently (March of this year). This breaks most distro rustc users as relatively few distros have shipped 1.51 into their normal release channels. The requiring of super recent rustc in a minor-minor version is somewhat surprising, and broke some of our build pipeline.

Is it possible to define the MSRV policy and maybe make the arbitrary feature optional, given its not required unless you want the fuzzer to map the types automagically?