@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)
I think just marking it as trusted is not good idea.
As I mentioned in the above request (https://github.com/D-Programming-Language/phobos/pull/2051#issuecomment-39527857), we have to consider the possibility of segfault in FreeBSD systems.

I agree with that `unpredictableSeed` should be safe (or trusted), but we should make `Thread.getThis` safe or trusted before marking `unpredictableSeed` as trusted.