@safe'ty of find and canFind

Change find's tests to validate usability of @safe.
Eliminate the trustedFindRR case by pushing it down a layer in the find stack of functions.
Mark unpredictableSeed as @trusted (@safe is a stretch due to all the casting) so that it can be used in @safe contexts.
Mark canFind's tests to validate usability of @safe (which lead to the unpredictableSeed change)
> My concern is pretty much as I stated in #2051; it's less to do with the safety
or trustability of _this_ `unpredictableSeed`, more about whether we could keep
that promise with other, perhaps more sophisticated implementations.  But that's
perhaps a problem to worry about if and when we encounter it.

Why would we ever want an implementation of `unpredictableSeed` that "may corrupt memory"?